IE7 link is hijacked by Dell Search

If I'm on site X and click a link to site Y, I get a Dell Search page with an error message similar to:

http://ad.doubleclick.net/adi/N3271.eBay/B3234323.4%3Bsz here are some similar sites.

It's not always "ad.doubleclick.net...".  It seems to depend on the link I'm going for.  This doesn't happen using my Firefox or Safari browsers. 

I've scanned my drive for spyware with Webroot Spy Sweeper for MSN and scanned for viruses with McAfee Security Center for MSN, etc., but none are found though the programs have the latest updates and defs.

However, a trial download and scan with AdawareAlert finds over 300 suspicious items to remove.  I have to buy the program to remove these items

Any advice?