Sorry, this is copied from another post so pick over what you want and ignore it if you don't like it.
"There were 2 causes of this and one I never found an easy cure. Here goes.
1. I had some USB printer, hard disk, or other device plugged in and on some Acers, HPs and likely other machines the machine turns on and does nothing. Fix? Unplug ALL usb devices and try again. Very annoying and does not help everyone but it is now welded onto my list of things to try.
2. I tapped the power button on my Vista Acer laptop. It shutdown just fine. I installed more RAM and it would not boot! It just was a blank screen. I removed the RAM and it booted the next try. YES I HAD PULLED THE AC AND BATTERY DURING INSTALL. But that wasn't it.
Fix? Don't tap the power button as it was set to go into hibernation. Instead do a real shutdown, power off via the windows menu. Then I removed the battery, AC, installed the RAM and then put in the battery, AC and it booted fine with Windows reporting the added RAM.
Very repeatable and I tried but could not fix this short of removing the hard disk to place on another machine to delete "hiberfil.sys"
The advice prior to this is good and I won't duplicate their advice but add my 2 entries.
In addition, research:
I am working on a friends disabled desktop. Its an Acer aspire M5641. Originally he said it would not boot into vista not even safe mode. I pulled the HD and scanned it on another PC for viruses (only found one) then downloaded a bootable vista repair disk to try to boot it from that. Most times when you try it gets to the "windows is loading files" message with the progress bar at the bottom it will make it usually 1/4 to half way across the screen and simply stops. The one time I did get this status bar all the way across the screen I got an error message similar to "recent hardware or software has corrupted file windows\system32\boot\winload.exe"
Why can I not get this PC to even boot form a windows repair disk?