virtualbox.org

I have Win98SE installed as guest on a WinXP machine. Is it possible for me to copy the .vdi for that installation, or some other file, to my Win7 machine, and just run the guest that way?

You can do that, but you'll want to make sure your VM settings are the same on the Win7 machine, otherwise Windows will likely bluescreen upon boot.

Now that version 4.0 has a convenient single-file virtual machine export/import feature, you should probably just use that.