Are you dual booting and Windows stopped working (booting) then here's the answer:
Okay, here's the deal, I've been multi-booting a few distros for a while now and have always had WinXP on the 1st partition of my 1st hard drive and the other distros follow it, both on the same hard drive and on my other two hard drives. Yesterday I went into WinXP so I could sync my PocketPC and upgrade some programs on it, everything worked fine and when I was done I rebooted back to Linux. Now today I went to boot into WinXP and after selecting the windows option on Grub (my bootloader of choice) like I always do it just sat at a blank screen saying "booting Windows', I thought great, either I was in for a re-install of Windows and a battle with getting my bootloader back how I liked it or just erasing the MBR and still having a battle getting Grub tweaked like I had it.
It then occured to me that Windows (and the Windows install disc) should be able to tell me what is wrong with my current installation ( I do give credit to those Redmond guys for this), so I boot up my PC with the WinXP install disc in the CD drive, sure enough it boots up and asks me if I want to repair an installation, I choose yes but it finds and fixes nothing. So I do what any lost person would do before thowing in the towel and re-installing, I ask for HELP. I type in help in the windows rescue prompt and a whole list of topics fly by the screen, and one stuck out like a gold nugget: FIXBOOT. I figure that is weird, whats the difference between FIXBOOT and FIXMBR?? Well I typed in FIXBOOT without hesitation and it spins for a few seconds and tells me I have a bad boot sector on my C: drive and asks me if windows should try to fix it. I type in the word "yes" with no expectations (I mean it is the Redmond boys afterall) and to my surprise it tells me it has fixed the damaged boot sector and I should remove the install disc and reboot to make sure it is working properly.
I reboot and find Grub in perfect working order, I move my cursor down to the Windows selection, hold my breath and hit Enter. Windows boots up perfectly and everything is back to normal.
Now that was the easiest fix to a problem that I thought was going to cost me a lot of re-installing/re-compiling/re-downloading etc.
Just thought I'd share, I've always seen people suggest the fixmbr option but I haven't seen this discussed before so I thought I'd let it have its 15 seconds of fame before someone tells me there are easier ways.