Fist things first make sure ALL of your drivers are up to date and your system is FULLY patched from Micro$oft.
Then it sounds like its time for some maintenance.
Here's some steps for a good maintenance routine.
1/ Remove temp files and other rubbish form your system, either with inbuilt Disc Clean up or CCleaner (www.ccleaner.com).
(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 > Top 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 (http://free.grisoft.com/doc/1) is good and free AV)
(A couple of good removal tools are Spybot (http://www.safer-networking.org/) and Adaware (http://www.lavasoftusa.com/software/adaware/))
ALso an on line virus and spyware scanner is Trend Housecall (http://housecall.trendmicro.com/)