Computer slow and CPU 100%

Okay, I guess this is where this should be posted? My four month old computer was fine until about a month ago, when it kept crashing. Turns out my memory was corrupted, and the Dell guy came out and replaced it and said there shouldn't be anymore problems. When I got on it afterwards, it started going really slow and the cpu was always running at 100%, which is something it never did prior to the memory replacement. I can't even run 2 IE windows because it goes so painfully slow, and every other program like WMP and Word just freeze and stop responding for like 10 minutes before they actually start to do stuff. I've tried some of the things suggested on other threads, and nothing worked. According to virus scans, I don't have any spyware or viruses. The processes with high CPU usage are the system process, csrss.exe, and various others that are constantly fluctuating, so its pretty hard to pinpoint what others, because its always a different process all the time taking up stuff. At this point, I don't know what else to try, and I thank anyone kind enough to help me!

Dell Dimension 3000
OS: Windows XP Home Edition with SP2
CPU: Intel Pentium 4 2.80GHz
RAM: 509 MB
Hard Drive: 80 GB

 

could it be a virus?...i dont know what to tell u..but reinstalling windows could be a last resort

 

http://forums.majorgeeks.com/showthread.php?t=35407