One friend has a continuing problem of her computer running very slow although it’s a relatively powerful laptop. Here’s what I found and what might help you.
Leave your computer on Sunday Night - I recommend this to run the Microsoft Security Essentials updates and scans that are scheduled. I assume you set these up. When you don’t run the updates and scans as scheduled, it may be trying to update during the day while you use the computer.
Memory – Check how much you have. It might be time to upgrade. In this case the computer had only had 2GB. We are going to upgrade to four. Unfortunately the 32 bit OS will not support the entire 4GB.
Windows Update – Check all the Windows Updates for problems. I found that Service Pack One was trying to continually download and update. There was a problem that it couldn’t install. So it might be continually downloading in the background. Another update would not install and all so we hid it – Right Click.
Shut Down at Night – Shut down your computer properly at night. This allows your computer to do it’s updates. If you just close the computer, it will go into sleep mode and may never update.
Task Manager – do a Ctrl-Alt-Del – find out what processes are running in the background. I found that HPNetworkCommunicator.exe was running in the background. I found some solutions online. http://goo.gl/sBMHr. You can also adjust MSCONFIG to stop some of the these programs from starting.
Security Updates – while you’re at it run Belarc and see what other security updates you need.
Viruses – do a virus scan to see if your computer might be infected.
Malwarebytes - sometimes finds Malware that a virus checker won’t find.
CCleaner – will clean up your system – thanks to Margery for the comment below. (Note – I ran this and found 455 items in my Registry that had problems and about 5 gb of space I could reclaim. The computer doesn’t boot any faster, but it’s nice to have it cleaned up.