Subject to the HDD not being physically damaged, it should be capable of being accessed by any other windows machine and allow a scan to be made for Virus and Malware infections, also while it is in the host machine you can always copy off your important files to disk after scanning, then if it proves to be a hopeless case a format and reinstall will not loose everything.
You should be aware though that in doing so you must scan the files carefully to avoid re - importing the original problem once again.
However if you have your original installation disk you might try an overwrite repair of the system files using that disk, this will preserve your data but unless your disk includes Service Pack 2 it will return the O/S to original state sans service packs and updates, which you will then have the task of reinstalling from Microsoft’s update service or if your lucky enough to have one, an SP2 update disk makes the task a little easier leaving only the critical update list to download, either way before you do any of this unplug your computer from the net so giving whatever your infected with no avenue to update, upload or download anything and once running do a safe mode scan, can send you details of how this is done if you need them.