Thanks for your answer!

Problem solved!

But to answer:

1) Yes I mean choosing Windows from the linux bootmanager that appears just after the startup.
2) I had nothing in my optical drive.
3) Logging into linux as root, I found out, that the linux installationprogram had placed a boot.ini file and som additional files in the windows partition.

4) Solving the problem: I I changed the files in 3) and bootet af windows vista repair disc (can be downloaded for free), it fixed the problems and everything works fine now!

5) Yesterday morning I hated Linux, yesterday evening I was fallen in love with Linux, so great! The only bug is, that the main menu keeps on crashing (gnome), but I've read that this is a common problame.

/Hans