I`m certainly no expert on PCs, but you may have bad connections to memory, HDD or other items. Whether these have corrupted data I couldn`t say.
If you feel confident, you could remove the battery, open the case and check all connections, re-seat the RAM and ensure the cooling ducts/fan are free of dust/dirt.
Another option would be to get a bootable CD click here to see if you can see the data on the HDD. You may also find these sites of help.
And if all else fails and your PC was shipped without a recovery disk, I think there was a class action around 2004 because HP were not shipping these disks so a search on this may help.