I first started having a problem with Vista hanging on start-up. I finally figured that out by process of elimination, the cause was FireThreat. So I uninstalled it and then had no more problems with hanging while booting up.

Then a few days ago, I installed a new program that C-net sent to me because I was a member of C-net. The program is called "C-net TechTracker". The program scans your computer and tells you all the programs on your computer that is outdated and then provides you the link to use to update the programs. It found 17 outdated programs on my computer. It gives you the choice of which ones you want to update and I only updated 13 of them, didn't need to update the other ones because I update those manually. I also made sure that my Windows was completely updated. I also ran Belarc Advisor to make sure I had not missed any critical security updates. I changed the settings on Cnet TechTracher so that it would not run in my system tray and that it would only run once a week to tell me if I needed any new updates. I don't know if any of this has anything to do with my shut down problems now, but since updating everything with Cnet TechTracker I have found 2 annoying problems. For one thing, my quick launch bar has changed--now I don't have the up & down menu with my quick launch programs, they are all spread out on the quick launch bar--and now what ever I am working on or when I am on the Internet, I have an up & down menu that I have to keep clicking on arrows to go back and forth between, say Word & the Internet. The second annoying problem is that Window's Media Player is now running in my system tray and I can't stop it from running there. Right clicking on it shows no settings in it whatsoever so that I can tell it not to run on startup. Next I used MSCONFIG to go to my startup menu and unchecked it there, applied changes and then the next time I started up my computer, Windows Media Player was right back in my system tray! Now when I shut down as you normally would, I get a slightly blacken screen with an error message in the middle of the screen that says: Explorer.ex-Application Error-The instruction at 0x768f8ed referrenced memory at 0x00000000. The memory could not be read. Click OK to terminate the program. Which I then click OK.

I have no idea what is causing this problem so I can not fix it. Below is information about my computer.

Dell Inc. Inspiron 531 00 (bought Dec 2007)
Windows Vista Home Premium Service Pack 2 (build 6002)
Firefox 3.5.1

Processor a
2.30 gigahertz AMD Athlon 64 X2 Dual Core
256 kilobyte primary memory cache
1024 kilobyte secondary memory cache

Main Circuit Board b
Board: Dell Inc. 0RY206
Bus Clock: 200 megahertz
BIOS: Dell Inc. 1.0.6 09/06/2007

1500.01 Gigabytes Usable Hard Drive Capacity
1332.24 Gigabytes Hard Drive Free Space

PBDS DVD+-RW DH-16W1S SCSI CdRom Device [CD-ROM drive]

HP psc 2175 USB Device [Hard drive] -- drive 6
SAMSUNG HD501LJ SCSI Disk Device (500.11 GB) -- drive 0
TEAC USB HS-CF Card USB Device [Hard drive] -- drive 2
TEAC USB HS-MS Card USB Device [Hard drive] -- drive 4
TEAC USB HS-SD Card USB Device [Hard drive] -- drive 5
TEAC USB HS-xD/SM USB Device [Hard drive] -- drive 3
WD 10EAVS External USB Device [Hard drive] (1000.20 GB) -- drive 1
Memory Modules c,d
1982 Megabytes Installed Memory

Slot 'DIMM_1' has 1024 MB
Slot 'DIMM_2' has 1024 MB
Slot 'DIMM_3' is Empty
Slot 'DIMM_4' is Empty

Local Drive Volumes

c: (NTFS on drive 0) 489.32 GB 386.46 GB free
d: (NTFS on drive 0) 10.74 GB 4.92 GB free
k: (FAT32 on drive 1) 999.96 GB 940.87 GB free

IDE Channel [Controller] (2x)
NVIDIA nForce Serial ATA Controller
Standard Dual Channel PCI IDE Controller

NVIDIA GeForce 6150SE nForce 430 [Display adapter]
DELL E228WFP [Monitor] (22.0"vis, s/n KU3117AJ1MML, October 2007)

Bus Adapters
Microsoft iSCSI Initiator
Standard Enhanced PCI to USB Host Controller
Standard OpenHCD USB Host Controller

NVIDIA nForce Networking Controller

Realtek High Definition Audio

USB Human Interface Device
HP psc 2170 series
HID Keyboard Device
Microsoft USB IntelliMouse Web
HP PSC 2170 Series (DOT4USB)
USB Composite Device
USB Mass Storage Device (3x)
USB Printing Support
USB Root Hub (2x)
Generic volume shadow copy
psc 2175

Virus Protection
avast! antivirus 4.8.1290 [VPS 081124-0] Version 4.8.1290
Realtime File Scanning On

Some Software that might be relevant--too many to list all:

Acrobat.com *
Ad-Watch *
Adobe Acrobat Version *
Adobe AIR 1.0.1 Version 1.0.1 *
Adobe Reader Version *
ALWIL Software - avast! Antivirus Version 4, 8, 0, 0 *
Andrea Electronics Corporation - APO Access Service (32-bit) Version *
Apple Inc. - Bonjour Version 1,0,6,2 *
Apple Inc. - iTunes Version *
Apple Inc. - QuickTime QuickTime 7.6.2 (1324) *
Apple Mobile Device Service Version *
Apple Software Update Version *
Auslogics - Disk Defrag Version 2.x *
Avanquest Software - Digital Line Detection Version 1, 0, 0, 2 *
Belarc, Inc. - Advisor Version 7.2v *
BVRP - Netwaiting Version 2.5.44 *
CBS Interactive - CNET TechTracker Version *
Google Desktop Version 5.9.906.4286
Google Updater Version 2.4.1441.4352.beta *
GoogleToolbarNotifier Version 2, 0, 301, 1654 *
Macromedia, Inc. - Shockwave Flash Version 7,0,14,0 *
Macrovision Corporation - InstallShield (R) Version 10.50 *
Macrovision FLEXnet Connect Version 3, 20 *
McAfee SiteAdvisor Version 2,9,0,0
Microsoft Corporation - Windows Defender
Microsoft® .NET Framework Version 2.0.50727.4016 *
Microsoft® .NET Framework Version 3.0.4506.4037 *
Microsoft® Schedule+ for Windows 95(TM) Version 7.5 *
Microsoft® Visual Studio .NET Version 7.00.9466 *
NVIDIA Corporation - nForce Network Test Utility Version 1, 0, 0, 6566 *
NVIDIA Driver Helper Service, Version 175.16 Version *
Pantaray Research LTD. - Engine Version *
Sun Microsystems, Inc. - Java(TM) Platform SE 6 U14 Version *
Sun Microsystems, Inc. - Java(TM) Platform SE 6 Version *
Western Digital - DLGDIAG for Windows Version 1.13 *

I am so sorry that this was so long, but I am not real computer literate and had no idea what might and what might not help that would enable you with enough information to help me. Any help would be much appreciated.
Note: Before I got this new computer, my old computer used Windows 98SE so Vista is like a foreign language to me! Also, I have a second hard drive (external) that is 1 T and I keep all my data on that drive.