Hm.
Bitlocker is a data encryption that is used by the fancier versions of Vista and 7, and I'm pretty sure that HP doesn't use it. Odd.
Well, yeah, you're most likely going to have to perform a clean install. Try choosing the "custom install" option, and install 7 on the same partition that Vista is on. If it will let you do that, it will save your old files automatically in a folder marked "Windows Old".