...I used to have an issue /w/ memory usage creeping higher & higher, then next reboot it would be normal & start over. I use Process Explorer to monitor it from the sys tray. The reason is, (mostly 3rd party programs & apps) don't return memory to the pool, so it climbs, leaving you less & less available RAM as time goes on. There's plenty of memory management tools out there but I prefer "UPHClean" from Microsoft, it works well in my XP. - Vista & 7 are supposed to have it built in so it shouldn't happen with those. If you use XP, I would recommend it, but only the original v1.6 - I had trouble /w/ the 2.0 beta - wouldn't recommend it. Weather you end up using the SAS or not, it should do the trick. It was originally developed to unload user profile hive to enable faster cleaner shutdowns (which it does), but, I find it works in real time to keep my RAM usage as stable & low as after a fresh reboot. - here's some reads...
http://forum.scottmueller.com/viewtopic.php?t=66
http://www.microsoft.com/en-us/download/details.aspx?id=6676