Try IE7 without any add-ons. If XP then you will find this under Start - All Programs - Accessories - System Tools. If IE7 goes back to normal then there could be an add-on problem.
Which anti-virus are you running? The anti-phishing feature can slow things down. You could try temporarily turning that off within IE7 and your anti-virus.
Clear your cache and see if this also speeds things up again.