Virtual CloneDrive works well.  http://www.elby.ch

Not only is it also free but it is easier to use than the Microsoft one.

Also useful for writing/creating CDs to/from ISOs is the ISO Recorder Power Toy (for WinXP). For free as well! http://isorecorder.alexfeinman.com/isorecorder.htm

It extends the Windows CD Recording Wizard to allow you to right click an ISO to write it to CD, and vice versa.