Master Boot Record (MBR) might be corrupt. If ur system was intact before and you are sure its not created by malware, you can try repairing the system with windows xp installer (your CD). to retrieve your important files therein, download "Knoppix" live CD Linux (could be DVD too, i m not aware of recent versions). Or, search for "live linux" distros (such as SUSE Live, PC OS linux or sth like that and so on- there are tons of them) from - www.distrowatch.com Then boot from the live linux and transfer your files to external media such as USB-connector storage (like flash drive, external hard drive etc) just a note, conflict between security related software also causes this type of problem. for instance, not all firewall and AV software are mutually friendly. Keep rolling...