You can do this via windows update but choose custom not express.

You can delete old files from your computer.

Defragment your hard drive.

Do a disk cleanup.

Click start/all programs/accessories/admin tools/disk cleanup.

Uninstall unessasary software you no longer use.

Make sure you have all latest Microsoft updates.

Download and run this:CCleaner - Home

Run antivirus scans.Regular.

Do you have the latest service pack installed?

IE7 installed?