Win 7 and IE8 is lightning fast for me, even on my little netbook. What I have seen slow it down is :
1. Antivirus programs -one I had to uninstall was AVGfree
2. Anti-spyware - Adaware rel time scanner was causing problems.
3. The Cox communication security bundle they give you free, had to uninstall.
4. Firewall programs - not set correctly.