One of the discussions on the Apple MobileMe iDisk forum is how to get iDisk mounted as a drive on Windows XP. While Apple has provided instructions on how to do this on Vista nothing was said about XP. The solution that worked for me was similar to the one that Apple provided for Vista and involves using the "Add Network Place" wizard.
If you’re using Windows XP, open the Explorer, click Computer, and then click “Map network drive” in the toolbar. Update: Apple has updated the MobileMe client for Windows. The instructions for Vista seem to work for Windows XP as well. Follow the instructions as above, enter http://idisk.me.com/[username] in the folder field, and click “Connect using a different user name.” Enter your MobileMe member name and password, and then click OK. That's it. You are done. No need to read the rest of this. Then click the link that says "Sign up for online storage or connect to a network server" to launch the "Add Network Place" wizard. Then Click OK.
Click to choose a network location and click next.
Then type http://idisk.me.com/[username] in the folder field, and click next.
Enter your MobileMe member name and password, and then click OK. This mounts your iDisk on your desktop, so you can work with it as you would any other hard disk.