I had the same problem. A VM would work, but it slows my 1 GB laptop down to a crawl. So I ended up having to upgrade to 2 GB memory and install Linux. I enjoy this A LOT better. But since that is not an option here is what I came across.
VirtualBox - Personally I like this one. If you have a Windows XP disc lying around this option is great. Especially if you have virtualization options on your processor.
SuperPreview - Though I don't like this one it is an option. I had Windows 7 installed because it ran better than Vista and XP but it gave me Internet Explorer 8. I tried this, and it only let me preview Internet Explorer 8. Maybe it was too early a beta or because I had a beta OS. Either way, I dont like this one just yet.
IETester - By far a great great program. Works perfectly in XP and Vista. Internet Explorer 6 and 7 don't work right with it in Windows 7. But if you have Vista this is the way to go. Multiple Internet Explorers don't work right under Vista but this does.
MultipleIEs - As stated above, this does not work in Vista, but it is an option for any Windows XP user out there. There have been known issues with this causing your OS to actually become more unstable, but I don't know for a fact if that's true. I just heard some reports about it.
Internet Explorer 7 Standalone - Though not applicable for this question others may find it useful to know that there is a standalone Internet Explorer 7. On my VM I have Internet Explorer 6 installed by default and Internet Explorer 7 as standalone. I would not recommend this as standalone Internet Explorer 7 is not perfect. MUltipleIEs + genuine Internet Explorer 7 or IETester is much better option.
BrowserShots.com - Browser shots is a great tool but somewhat limited. Basically it takes a screenshot on literally hundreds of browsers and lets you view them. The thing is it is mighty slow, and you don't get a full screen preview.Not a great option.
Meer Meer - I heard about something from Adobe Labs called Meer Meer a while back, but I was never able to actually find it. I believe meermeer.com actually takes you to Adobe Labs' website too. But it looks like a great tool complete with Onion Skin Overlay and the works. Like Super Preview but on the web instead of local. If it is released, definitely would check this one out.
Overall, I say IETester is a great bet. Anyone running Windows 7 however is left in the cold. I tried every option out there and none of them worked for me except for Linux host and VirtualBox XP guest. That's my 2.5 cents, though.