Is your pc pausing or freezing for a little bit while you open up a new application or switch between email and browsing the internet? Simply try a disk defragment for your harddrive, it's located under start-->all programs--> accessories--> system tools--> Disk Defragmenter. Just run that, then restart and see how it runs (if you haven't defragged in a long time, run it 2 or 3 times).  Also, do nothing with the pc while it is defragging, so you may wish to wait and perform the defrag at a time when your pc will not be needed.

Also run the Disk Cleanup utility, it's also located in the System Tools area of the start menu. It scans your pc and removes any files you don't need on your pc.

Maybe your pc is out of date? Try running windows update, start-->all programs and at the top there is a globe with the windows flag on it that says "Windows Update" click on that and run update to keep your machine up to date.

Scan your pc with your antivirus client to make sure your safe and clean from any viruses. When you do a virus scan, it's also good to let it scan while no other functions are running, so do it when you can step away from your computer for about an hour.