Have you tried booting windows into safe mode (press F8 as soon as the BIOS screens disappear) to make sure you haven't got a corrupted driver in Windows? If windows is almost loading that is where I would look first. Sometimes if windows gets corrupted even safe mode refuses to start. Try booting from a MEMTEST disc (see below) and let it run for a while. If it runs without errors it is likely to be a windows fault.
Another thought is how long is it since you replaced the BIOS battery on the motherboard - they should last 2-3 years but I have had them last less than that and you get all sorts of odd problems which look like BIOS corruption.
If the battery is OK and you are still having problems I would try the following sequence:
On another computer download MEMTEST86+
and create a floppy as per instructions.
Switch off and remove the power cable. Strip your system to bare essentials:
- CPU + FAN
- Single stick of memory
- Onboard graphics (preferably if you have it) or graphics card if you don't have onboard graphics.
- Floppy disc drive
Unplug everything else except the PSU connector and don't forget your USB devices etc. plugged in at the back. (Make a note of where everything goes if you aren't sure)
Boot up and go into the BIOS (usually press DEL quick at the first sign of life). You may want to make a note of any BIOS customisations you have made before you proceed further.
Switch off and remove the power cable. Remove the motherboard battery and move the link to clear the BIOS memory to its clear setting. Wait a few minutes then move the link back to normal position and replace the battery. Now plug in and switch on. In the BIOS find the option for default/factory settings and restore those. Disable any peripherals you don't normally use (such as game ports/serial ports etc).
Also find the option to quick boot and switch it off - this will force the BIOS to do a more thorough POST.
Now switch off - leave it for a couple of minutes. Insert the MEMTEST+ floppy disc and switch on again .... if it gets through POST and tries to boots from the floppy it doesn't look like a hardware fault on the basic system. Leave it running over night.
If there are no MEMTEST faults next morning it is likely that basic hardware is working fine. Now add back your hardware bit by bit and test after each one (run a few cycles of MEMTEST each time). Don't connect your hard discs until last (otherwise you may have to activate windows again because of massive hardware changes).
If you still have trouble booting into Windows at random places it may be a capacitor on the motherboard - check all of them visually with a torch (the ones that cause problems usually look like small tin cans). Check there is no discolouration on any of them. A number of years ago Athlon boards were plagued with duff capacitors (there was a dodgy batch distributed to all manufacturers and a lot of boards got recalled). I was suffering a similar problem (except sometimes my system refused to power on at all if poswer had been off all night). In the end I gave up and built a new system.
If the worst comes to the worst try eBay for a replacement mobo - otherwise you might have to bite the bullet and get a new system!
Another problem solving approach would be to do an image backup of your system partition and then try a quick fresh reinstall of windows - if that boots you know it is the original installation - if not you know its probably a hardware compnent.