SL & Parallels 4: "don't have rights to use this virtual machine"

Parallels is installed under an admin account and requires admin access. If you want other users to use Parallels then: a: Create managed accounts for other users and assign access to Parallels; b. Install Parallels in each users /Home/Applications/ folder; c. Place the /Home/Documents/Parallels/ folder in the Shared folder; d. Use solution (a) but let each user do their own individual XP installation on their own pseudo-drive in their own user account's Documents folder. I think the latter makes good sense since it prevents one user from making changes to another users Windows system and software.

The purpose of the Shared folder is to simply exchanging document files between users, not as a way to share applications and all their associated files. Using managed user accounts is how you should match individual users with access to applications.