You've got spyware changing it. Go to http://download.com and download adaware.

Make sure you update the spyware bases before you run it. After you run it, you should be set. I usually have mine do a quick scan of my registry on every login. Saves time and effort. Hope this helps!

Definitely sounds like spyware/malware. To add to TKS' suggestion:

-Read through the threads in our Security forum. Some of our members have posted a lot of great information concerning (free) utilities and procedurs to use to get rid of "hijacked" homepages, unwanted pop-ups,and the many other manifestations of rouge malware programs.

- Immediately after cleaning your system with the recommended removal utilities, run Windows' Automatic Update feature to download and install Microsoft's latest security patches and bug fixes for XP, Internet Explorer, Media Player, etc. Many of the latest patches fix some of the security holes which allow rouge programs to get into your system.

- Keep your removal utilities up to date; updates for these programs are released very frequently. Scan your system weekly (at least) with the utilities; you'll be surprised how quickly your system can get reinfected.

- Unless you absolutely can't live without IE, use another Web browsing program. IE's tight intergration with the Windows operating system itself makes it a greater risk in terms of intrusion that a non-MS browser.