STEP 1 : Post HijackThis Log File Content
If your system is running very slow or giving random problems, there are chances that its infected with a virus. Download HijackThis from the links given below and scan your system with it. It'll generate a log file, copy the content of the log file and post here.
STEP 2 : Fix Suggested Entries
To fix the suggested entries, Boot your Windows in Safe Mode by pressing “F8″ key at system startup and select "Safe Mode" option. Run HijackThis again. Then select the entries and click on “Fix checked” button.
Also don't forget to scan your system with a good anti-virus and anti-spyware.
You can also visit following tutorials to speedup your Windows:From: Tweaking with Vishal