I used to have Ubuntu64 on this machine along with WinXP (on AMD64 3000) until the routine win C: reformat after which Ubuntu was inaccessible. I guess boot partitions got corrupted? When I tried to install Ubuntu again it crashed during installation. Any idea what I might do to get Ubuntu back?

When you reinstall windows the mbr (master boot record) is re-written and you loose the boot manager (usually GRUB). The data and partitions are there, but the boot record only has the information to boot windows. You have to reinstall grub mounting the partition with the LiveCD or use an excellent tool called Super Grub Disk (SGD) which is easily found googling. SGD lets you create a diskette or CD that boots and give you tools to revover the lost grub.

Good luck!