When I startup the computer, I can't access the BIOS or go into the boot menu. I have two OS installed for some reason, and the top one in the selection list works luckily, because I have no response there. But when it reaches the login screen, I can't use the keyboard or mouse to login.
I have three HDDs, One running XP and two running 7.

500GB WD HD - Running Windows 7 (Preffered)
1TB Segate HD - Running XP
1TB Segate HD - Running 7

The two TB drives will work perfectly once they reach the login screen, but I only want to use the 500GB one as my Os, because I was using it for the last two years before my Motherboard broke. I can't access the BIOS or anything prior to the login screen on any of them. It will load whichever one is 'master 0', and I switch the ribbon link accordingly to boot the correct drive. So is there a way I can change the BIOS settings from either XP or Windows 7?

commented: Save our data in a hard-disk. break your HD partion. after then re-setup your pc. +0

Does your mouse and keyboard work at all? If not thats the first problem you need to sort out - make sure your using the ones that came with the computer, try them in different ports, if it's using a round looking port, buy yourself a cheap USB keyboard. When that is fixed you will want to boot which ever OS works, so the one on the 500GB HDD? Then copy over any important data you have off the other 1tb drives, then format the 1tb drives to remove the OS. It will confuse your PC with 3 OS's installed.

Hope this helps - mat

Are your keyboard and mouse usb? Does your motherboard have ps2 keyboard and mouse ports? If so, you will need to borrow a ps2 keyboard to go into your bios with so that you can enable usb legacy support.

maybe this will help ,its been awhile but it thought this could be turned on ,if you are using win7 try this ,not sure if it works in win8
click the little red icon in the lower left corner to turn on onscreen keyboard
Click Here

I'm going with a "broken" BIOS. USB [and PS2, if available] hardware support should just work in BIOS. Perhaps you need to flash.

I believe I had this problem once upon a time, and the solution for me was what Rik from RCE suggested.