It's running XP Pro, everything seems to be connected fine. Recently I'd opened the case up to remove an old CD-RW drive that was malfunctioning, but I fixed the drive and returned it back to its original spot, and it worked fine. But today when I turn on the power, it shows me the Asus motherboard boot screen, then the XP Pro loading screen, which appears with the moving 'loading' bar for a good three to five minutes, at which point it goes back to the motherboard screen. I've tried removing the drive, and turning the power supply off/on then attempting to reboot. Still, it won't move forward past the Windows XP screen, at which point it goes back to the motherboard screen. What could be the problem?

Try, unpluging all devices from the computer such as usb printer, scanner and leave the bare essentials, monitor, mouse, k board and tower hooked up. Try rebooting. If the problem persists, try to boot into safe mode and use System Restore to roll back to a date where it was working.

the soraci virus also causes machines to "loop" on boot. this can be sorted by connecting the hard drive to another machine as a slave(set the drive's jumpers accordingly), set the bios to see it then don't open it but scan it with a good virus scanner to sort any virus out. then set it back as before and return it to its original machine and if needed reinstall (overwrite don't format) to repair xp. if you need to repair the boot sector first boot from xp install cd, then run the repair function. at the c prompt type FIXMBR return, then FIXBOOT return.