Hi Claudia,
Repair Mac OS X file permissions, as some Parallels Desktop files may have wrong permissions:
Open Disk Utility (/Applications/Utilities/).
Select your Mac OS X startup volume (Macintosh HD by default) in the column on the left of the Disk Utility window, then click the First Aid tab.
Click the Repair Disk Permissions button and restart your Mac and check it out how it works.