Yeah, UAC is disabled. I tried many different things. From activating the admin account to manual registry cleans. Nothing...
Last night I tried to start the OS in safeboot mode and install - That's what did the trick!!!
If somebody else ever has this problem (win 7 ultimate 64 bit):
1. click the windows start button
2. type msconfig
3. go to "start"
4. check "safeboot: minimal"
5. restart windows (it will do a safe boot now)
6. log in and install fs2crew
7. open msconfig the same way you did before and uncheck "safeboot: minimal" under "start"
8. reboot
9. DONE!