Some programs can lag your computer bad depending on what they are. To see which programs are connected to the internet right now type this:start -> run -> CMD -> netstat -b
This list shows I'm connected to instant messaging programs. Try closing all your opened programs. If there's any that linger, then it's likely something malicious or super annoying like msngr.exe, windows messenger that seems to not be killable through control-alt-delete, or editing startup.
First thing is to edit startup. Click the startup tab and uncheck any programs you dont want lingering when you turn on your computer. To do this use msconfig and click the startup tab.start -> run -> CMD -> msconfig
Next click services and organize by manufacturer. Remove any of them you don't recognize, not made by microsoft. If you're unsure search google with the name. Same goes for startup items.
You can start, stop and disable currently running services, and get a breif description of what they do with:
Some programs wont let you remove them.The best program I've found is delete on boot. You just drag and drop the program for removal. When your computer is restarted it will remove it. Alternatively you could use a boot disk and delete it in dos. Go to the file with "cd C:/filename" then "del x.exe".