Zefiris:
When you say your computer failed to boot, what, exactly, do you mean?
Did you get a black screen, or was there an error message?
If your hard disk is failing, you'll get a message saying "boot device" or "boot sector" not found. If one of Window's boot files is corrupt, you should also get a message.
No message at all can be a BIOS chip fault, and this can ususally be fixed by flashing it with a copy you download from the manufacturer.
Often, as previously mentioned, programs will add a "boot once" link to the start-up queue, and (especially anti-virus and anti-spyware and Microsloth upgrades) these run before you ever get a "Welcome screen" and so all you see is black. The new program load can take several minutes.
The key is to watch your hard disk L.E.D. and check for activity. If it's blinking, you're loading software, so let it run. Once or twice in the past I've almost pulled the plug, thinking I had a virus that was reformatting my hard drive. Then I remembered I'd installed new software.
You can run a disk test to make sure the problem isn't a weak copy of your boot loader, of course, and I'd recommend this if it happens again.
Good luck.