Problem with Not enough Quota

I have a Dell Dimension 8250 running under Windows XP Pro SP2.  It has 512MB PC1066 RDRAM and a 120GB 7200 RPM hard drive, Zip drive, diskette drive, CD drive and DVD drive.

Until a couple of months ago, the PC ran OK.  Now I keep getting messages "Not enough quota is available to process this command."  I've looked through the posts on this issue, but still don't quite understand the issue.  I am and administrator and the only user of this PC. 

In thinking back, this problem seems to have started about the time I upgraded my security software to Trend Micro Internet Security Pro.

I have run online security scans from Trend Micro, Symantec, and McAfee to check for viruses, etc. but nothing was found.

Initially I suspected low memory was the problem as when I would open a couple of programs at the same time, sometimes the loading screen would go blank.  But that sometimes happens now when nothing else is open and I try to fun a program. 

I'm not sure if I have a PC memory failure, a bug that has gotten past the security software, or something else.  When I tried initially to set up my network about a year ago, nothing would work, so I had the Geek Squad come help out and he found a Trojan horse on the 8250.  After that things worked fine until now.

Any suggestions will be greatly appreciated.