Just by leaving my computer on, my notebook will slowly sap all 4 gigs of memory from my RAM. Basically, at Task Manager, I am left with zero (0) "Free" MB's of physical memory. I call this "RAM creep". I call it something else as well, but my wife says it's better not to.
What operating system are you using? Is it up to date with fixes/service packs? What odd/unusual/3rd party software do you have installed especially with kernel components? Are all those bits up to date?
At work I have a Dell machine which will absolutely not run Windows XP, but it's happy enough with Vista. (Neither Microsoft nor Dell were interested in troubleshooting.)
Windows XP is not the only operating system I've experienced memory leaks with this year, but you're probably not running the server operating systems I deal with professionally.
Bottom line is if the problem is too expensive or difficult to troubleshoot, you'll have to reboot more often. Yeah, that's professional advice. Operating systems should
stay running without leaks for years at a time, but few do.