If you are in windows now try going to Accessorries>System tools> System Restore. In system restore you will choose a date that is before you started having these problems. If you did not have system restore "ON" ( You want it on in the future ), you could try putting your original Windows disk in like you are going to install the OS, but choose Repair instead. This will not erase you files or programs, it will just re-install your system files. Try those first.
If you are stuck trying to get Windows working again, when you get to "Start Windows Normally", choose "Safe Mode" instead. Windows will look a little strange. You will probably see a lower resolution screen, but that is normal in Safe Mode. In Safe Mode you can still use system restore. Safe Mode is a stripped down version of Windows.