Thanks for the update!
I have installed CrashPlan on both Windows XP and Windows 7 computer. I previously created a mapped network drive on both computers and CrashPlan could normally see the mapped drive as if they were my local drives when I clicked on the "Select..." button in the Folders tab of the CrashPlan window.
Here is a screenshot of CrashPlan installed on my computer, showing that two mapped network drivers are accessible by CrashPlan:
I am not sure why CrashPlan on your computer cannot see the mapped network drive. Note that I didn't use the special instructions for creating a mapped network drive you provided a link for, since I don't see how that is actually needed. Any user-created mapped network drive is fully accessible for all installed programs, including CrashPlan. As mentioned, I tested this on both Windows XP and Windows 7 computer.
Anyway, in case your CrashPlan still cannot access the mapped network drive when you click on the "Select..." button in the Folders tab, you can use another method of saving backup to another computer.
Here are the instructions:
1. Install CrashPlan on both your Windows XP and Windows 7 computer
2. Use the same CrashPlan account for both computers
3. Start CrashPlan and click on Destinations
4. Click on the Computers tab
5. You should then see both your computer names listed there. Click on your other computer name and then click on the Start Backup button
6. The backup will start and use your other computer to save the files over the network
If even that doesn't work for you, I can remotely connect to your computer and check the issue.