Windows 7 does a pretty good job of detecting applications made for older Windows versions and offering a compatibility fix. If you've got an app that just won't run, though, take a dive into the Compatibility Mode's deeper settings for a solution.
Mysticgeek writes at the How-To Geek's home away from Lifehacker about the ins and outs of Compatibility Mode, Windows 7's hidden-but-awesome feature for making older programs work more seamlessly inside itself. Mysticgeek includes a fix Windows 7's overeager installation checking in his review, but also shows how to find online patches and fixes for apps that Windows 7 claims not to support, and fine-tune how certain apps are run.
For Windows 7 users requiring access to older apps, it's a fine guided tour of a feature that doesn't just jump out at users and explain itself.
Using Program Compatibility Mode in Windows 7 [the How-To Geek]