There're two things you can try. First, when you power up, try pressing F12 a couple of times and see if that doesn't give you a boot menu. That works on all my PCs. Second if you can boot to Windows, insert the install DVD, and it should start the install program. If you're unable to boot to normal Windows, you could try Safe Mode by pressing F8 continuously upon booting up.
Good luck.
So I'm trying to reformat my HD, and I'm having quite the time doing so. I have borrowed and Windows 7 disk from a buddy of mine and was going to reformat my HD and then buy a key online, but here is my problem:
I put the disk into my DVD Drive and restart, I immediately go into BIOS and make sure that my DVD-RW drive is first choice in boot order, it is. Upon restarting, it just does a normal boot process, it doesn't give me the option to boot from disk.
What to do?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic