What is probably happening

What to do

Causes can include: using Windows Me/98/95/3x, not enough RAM, fragmented pagefile, hardware problems, improper performance settings, and many other things

Look for the obvious, first. Upgrade to Windows XP if you can. Replace your RAM, and get more of it. Check your pagefile settings, check connections inside the machine (connectors do come lose), check your performance settings, check for IRQ conflicts.