|"when I switch on my computer it goes through post and starts to load windows"|
While the above is happening, start tapping the F8 key in an attempt to get into Safe Mode. Your post seems to suggest that the "Del" key is your usual way to access Safe Mode. This is not the case. As pointed out in a previous reply, the "Del" key is more than likely used to access the BIOS.
If you can manage to boot into Safe Mode, it may indicate that the problem could be a result of a corrupted device driver, or some other software (which includes basically anything that's been coded by a programmer, e.g. viruses) that is set to run when Windows boots up. This can include instant messengers, firewalls, anti-virus, and an abundance of useless garbage that software companies insist must start with the OS (Quicktime is one such offender).
As alluded to above, older motherboards don't support USB devices outside of Windows (or whatever OS you use). In other words, USB keyboards are not functional until Windows has loaded the USB drivers. Newer motherboards should have onboard support for USB.
If we had more information about your system, we could be more specific with the steps we'd take to diagnose the problem. Without at least a motherboard make/model, we can only give generalized suggestions, some of which might not be appropriate to your system.
If you can open the case, the motherboard should have its model number printed on it. This shouldn't be necessary if it's a name brand computer (HP, Compaq, etc). For a name brand machine look on the outside of the case for a model number (we can determine the motherboard from that).
Do you know if the BIOS is set to boot from CD/DVD before the hard drive? If you can't access the BIOS (even after using a PS/2 keyboard), you won't be able to change the boot order. That means not being able to install Windows, should you manage to format the drive (without the ability to boot from CD/DVD, formatting would require removing the drive). It also prevents you being able to use bootable diagnostics disks, such as Memtest (to diagnose possible memory problems) or hard drive diagnostics tools. If you have a purple PS/2 port at the back of the machine, beg, steal or borrow a PS/2 keyboard and confirm that the BIOS can be accessed.
If you can boot from CD/DVD, there are a number of bootable diagnostics disks available (some Linux-based, some DOS), that can diagnose the various hardware components for faults/damage (Linux-based disks should give you access to the data on the hard drive, for purposes of backing-up). Memtest can be downloaded in the form of a bootable disk image (which can be burned on another computer). Seagate hard drive diagnostics works for most hard drives, and can be downloaded as a bootable image file. These bootable diagnostics may not be needed, providing you can boot into Safe Mode (with the F8 key).
If you have no luck solving this beforehand, post back with the computers make/model, or motherboard make/model. Either of these items is the very least system information we need to know.
Please let us know if you found someone's advice to be helpful.