The file or directory is corrupted and unreadable

It could be a virus, but I wouldn't think that a virus would single out the file types you've mentioned.  The "I Love You" virus a while back affected FoxPro's .sct files.  But, that wasn't in the category of a .pjt, .spr, or .frt file.  Keep in mind the .pjt and the .frt files are actually memo field files attached to the .pjx and .frx file tables, respectively.  So, a pattern would be that you are having issues with getting your work saved to memo field files.  The .spr files, of course, are a byproduct of screen generation that comes from the initial .scx screen (table) file with its associated .sct (memo field) file.  I wonder if the .sct file is corrupt in the instances where you have corresponding .spr file problems after the screen generation process.

If you are having other issues on that WinXP PC with some devices and their drivers acting up, that could cause issues with overwriting memory spaces that FoxPro is using and then the results will be corrupted in selected spots.  However, it seems you would get other symptoms of a problem in your FoxPro sessions long before you see corrupt files occurring.  It still seems that you have an issue as things get saved where they seem to get saved incorrectly.

Possibly see if removing your current Fox resource file from your active sessions helps to calm down your design environment.  Just delete (or temporarily rename) the foxuser.dbf file.  If that gets corruption, unpredictable results come up in very unusual ways.

Possibly you have some corruption in your project itself.  At least try to clean out past old work in the project by choosing the Project menu choice and the Pack option at the bottom of the list while you have your Project open on screen.  That will at least assure no old work is still in the picture, even though those records were marked for deletion.

I just cleaned up a laptop yesterday for someone where his college kid had taken very few precautions to ward off spyware attacks.  There were all kinds of rogue processes running in his Task Manager that were stealing precious RAM and CPU cycles from him.  There's no telling how they might have affected other running applications.  Anyhow, I first installed the Personal Edition of Ad-Aware that discovered and allowed me to delete most of the junk that browsing the internet introduces to the computer in the way of programs, new files, directories, and registry entries.   There were about 500
entries in the first scan-- that is the most I've ever seen.  Whatever pieces required to be memory resident stole about one third of his total processing power.  Anyhow, I recommend that you download and install and USE, if you don't already, the following spyware/malware removal tools.  What one misses the other one catches.  They both catch different things and together, you can get some piece of mind if you browse the web pretty regularly.

Ad-aware 6.0 build 181
http://download.com.com/3000-8022-10214379.html

Spybot - Search & Destroy
http://download.com.com/3000-8022-10122137.html

If you think McAfee is just missing the problem and you want a second opinion, try downloading the free AVG anti-virus software package and letting it scan your PC to be assured you in fact are clean from virus signatures.

AVG Free Edition
http://www.grisoft.com/us/us_dwnl_free.php

Other general references specific to FoxPro:

HOWTO: Troubleshoot to Resolve Suspected Corruption
http://support.microsoft.com/default.aspx?scid=kb;en-us;193952

INFO: Possible Causes of Data Corruption in Visual FoxPro
http://support.microsoft.com/default.aspx?scid=kb;en-us;264867