Yesterday I tried to encrypt my whole hard drive, but the truecrypt pre-test failed everytime.
After entering the password it didn't boot, just showed the black screen with the blinking letter _
So what was different this time? Before I reinstalled Windows everything worked fine.
Now I remember, this time I didn't use my own Key, installed and used the win7 loader by daz.
The loader seems to install a custom bootloader so it can work.
OK...win7 loader by daz + Truecrypt = error
Now I installed Windows once again, used my own product-key instead of the loader and tried again, works !