Sounds like time for some serious maintenance.
(I'm going to assume that you are using XP even tho' you didn't mention.
Most of these steps will work with any Operating System)
Here's some simple steps for you to take.
1/ Remove temp files and other rubbish form your system, either with inbuilt Disc Clean up or
CCleaner.
(Disc clean up; open my computer > right click your C: drive > properties > Disc clean up button on general tab. Let it run and select everything)
2/ Defrag; open my computer > right click your C: drive > properties > Tools tab > Defragament now button.
3/ Error checking; open my computer > right click your C: drive > properties > Tools tab > Tope button > check now. Check BOTH boxes, OK the error message and reboot.
4/ Page file; Right click 'my computer' > properties > advanced tab > Performance settings button > advanced tab > Virtual memory (near the bottom). To make things easier for you select System managed size > click set > then OK's all the way out again. (
I suggest 'system managed' as there is no consensus on min and max sizes to set)
5/ To make sure everything is running fine also run both anti virus and anti spyware apps (make sure that they are updated first ;)) (
AVG is good and free AV)
(A couple of good removal tools are
Spybot and
Adaware)
ALso an on line virus and spyware scanner is
Trend Housecall
Hope all this lot helps;)