I have three volumes - on C:\, there's nothing, on D:\, my Windows XP, and on F:\, I've got Windows 7. BIOS is always booting WinXP. How can I tell BIOS that I want run Win7? I tried to open OS choices menu, but it still writing:
Windows XP Professional
Windows XP Professional Setup
Three choices. Well, hundreds, but I will offer you three.
#1 Watch your BIOS as it boots up. There will probably be a very early message telling you to press one key for setup and the other for Boot Menu or BBS menu. Which key varies heavily from board to board.
Press that key. The BIOS will continue running, then offer you a list of all available boot devices. Pickest thou the second one, and 7 will come up.
#2 Press the key for setup in BIOS first. Go to the Boot setup section and select your Win7 disk as the boot device. An alternative - swap which controllers the two disks are on; by default, it boots off the lowest-numbered drive on which it finds boot information.
Then, when Windows 7 is up, have piles of fun with editing your Boot Configuration Database. I strongly recommend downloading and reading about EasyBCD. You add the XP boot to the booting list available as Win7 starts.
While this could be done in the other order, it's usually best to start the boot in the more recent OS and add the option to boot the older OS.
#3 Install a third-party boot manager, and use it to pick which drive you will boot off of. More complex.
Strong Warning: Messing about with boot configurations as I have described above presents a small but finite chance of messing up either the Master Boot Record on a drive or the Partition Boot Record on the partition. If you have the original XP / Win7 media, it is my personal opinion that repairing these problems is easy and reliable. Your mileage may vary. Back up your data. No warranty, express or implied, is provided.