Malware can do that, but first of all, in Internet Options > General Tab, delete everything in the Home Page area, click Apply then OK, then try and add your preferred Home Page.
If that fails, are you using any security software that prevents the Home Page from being hijacked? Something like Spybot Search & Destroy, Spywareblaster, or some anti-virus scanners might have that option. If so, turn that option off.
Then update and scan with your anti-virus scanner.
No matter how many times I change the home page in Tools, Internet Options, IE7 always goes back to "about:blank" for my home page.