This is usually the result of subsequent installs of new software that adds startup items as well as services that startup at boot time resulting in a slow boot and consequent demands on system resources that cause overall slow performance, you can use the clean boot article here How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7 to help determine which program(s) and or service(s) may be causing the issue. The article suggests adding the items back in half at a time, personally I would be a bit more selective and only add them back in if needed, or as required.
I had the same experience as JFT's. Tried to serach "clean boot" here, but found nothing. Would appreciate if Trouble could kindly advise how to conduct a clean boot under Windows 7 or refer to the relevant articles. Thanks a lot in advance.