The wacom tablet driver does not work with pressure sensitivity on either Windows 7 or XP, when applications are run inside a sandbox.
I used to have this adjusted in Sandboxie so it would work on XP, but had to redo it for Windows 7 because certain names changed. Now I'm sharing so others might benefit as well, and because the "solutions" thus far posted here don't work for everyone or even at all.
The solution is quite straight forward, though it took several iterations of running the Resource Access Manager to figure it out.
Add these lines in your config file for Windows 7: (series 6 wacom driver)
In Windows XP: (series 4 driver)
The last line for XP may be optional. Test it out.
This is ALL you have to do for both platforms. No need to allow hardware access, which doesn't help anyway.