If you have another PC nearby, take the HDD out of the PC and install it as a Slave drive in the other one, boot up and copy over the missing file. If you have a Windows XP Installation CD, perform a repair install. If you have any bootable media with a file manager, recovery software and Internet access such as a Linux distro, Winternals Admin Pak 5, Reatogo Builder or BartPE, boot to it and try to recover the file or obtain it from some place and transfer it to the HDD. Once you're back up and running, run Ad-Aware SE and Spybot S&D, update them and perform full system scans to remove any malware. Then install a freeware anti-virus such as Avast or AntiVir, update and perform a full system scan to remove any viruses/trojans/worms, etc.