Try a bit of general housekeeping to start with ...
Check your system for malicious code using an up to date Anti Virus program.
Check your system for spyware and adware using a checker such as Spybot.
Remove unwanted files and applications by running Disk Cleanup.
Check your Hard Disk for errors using Check Disk.
Defrag your Hard Drive.
Download and install the latest drivers for your system video and sound cards.
Panda Online Scan
http://www.pandasoftware.com/actives..._principal.htm
Spybot
http://www.safer-networking.org/