There is no such thing as Vista SP3. It only went up to SP2. In any case, the Vista disc version must match the installed version in order to do a repair.
Otherwise, you can try an over the top install, but then you'd need to reinstall the SP later.

For UBCD5, you would probably need to burn the image to a CD or DVD (instructions are in the guide), since your system may not support booting from a flash drive.
I'd connect a flash drive or external USB hard drive to the computer first, and then boot from UBCD5.
At the boot menu, choose Parted Magic and allow it to boot to a Linux desktop. No installation required. The file manager is similar to Windows. Just copy your data to the flash/external hdd.

I ran harddrive, memory, almost all of the ones on dell diagnostics. passed
Personally, I would still run the hard drive manufacturer's diags. I'm not sure how thorough the Dell diags are.