You didn't say what operating system you are using (DOS 6.x, Windows 95). In my experience this usually means the drive is dieing.
Try running SCANDISK to see what errors it finds. This will only be available DOS 6.2 and above.
If you have data that you don't want to lose, make sure you back everything up.
I recently worked on a Windows 98 machine that failed to boot. We reinstalled Windows and after one boot, it failed. It turned out that the hard drive had a bad sector right where the BOOT files are installed. So it would corrupt the boot files every time. That drive gave "Error reading Sector" messages.
Let us know where this gets you and more info about your system.
Yes, I agree, be sure you have a backup then
run scandisk and be sure to run the surface scan. I believe this option marks any problem areas of the disk as unusable. Afterwards, the error message should not appear again.
You can also run chkdsk.exe if you have an older Dos version. It is not as performant as scandisk, but it may get you able to boot again. But you should still get scandisk and run it with surface scan and make an undo disk. It may be just a FAT problem (a system crash may have caused a cross linked entry).
Data error reading drive C?
Assuming that the drive is setup correctly in the system BIOS theres only another posssibility
Bad sectors on the hard drive
Download the manufacturers hard drive tools & run them. If it fails the diagnostics replace it
Unless you are loading DOS from your A drive - If you can run DOS, your C drive is booting.
If you have a bad cmos battery your drive setup may have reverted to a type that, while it can read the boot sector, and possibly other items, like DOS, that are stored on the first few sectors of the disk, the addressing doesn't match correctly as upper sectors are addressed and the software reports a data error. You need to know the Head/Track/Sector information for your disk, 'cause most of the older DOS machines don't have the ability to get that information from the drive like the Win-95 PNP machines do. Most of the drive Mfgr's web sites have that info available.
I tried running SCANDISK and got the error message was "data error reading FAT on drive C" right off the bat. I then did CHKDSK and got the message "c:\qtinstal.exe is crosslinked on allocation unit 18,302". I stopped there, having no clue what this means. BTW, I have Dos 6.2 and win 3.1.
To put it in simple terms when you store a file on the drive it is split in to several pieces all over the drive. The index keeps track of where all these pieces are so the file can be reassembled. Crosslinked means the index show two different files using the same bit of data. While that happens a fair amount of time when everything is right, the error reading the FAT would indicate that the drive is going bad. This is the index. There are two of them. If they are not identicle, you will get an error. If a part can't be read, you get an error.
If you are still able to read the drive, you can backup the data. But the drive is most likely finished. You could attempt a rebuild. But you risk losing all of the data.