Try doing what Joxley1990 said. Try to think if you recently installed some new kind of hardware or software. This may have caused your hard-drive to be inaccessible. Also, you may have gotten a virus, or some other type of malicious code on your comp. Try running a anti-virus program in safe mode and seeing if it detects anything. Also, you might have a bad case of spyware/adware, that may stop your computer from booting normally. If all else fails, try looking for a recovery disc, or your Win XP cd and reformat your computer.
Rig -- Emachines T5026, 3.06ghz P4, 300W Bestec PSU, 160 sata hd, 512mb dual channel 3200 ram @ 3-3-5-8, ATI Radeon 9250 pci, dual layer 16x dvd+/-rw.