|unmountable boot vloume error|
This generally means you have a corrupt boot sector (but can mean hard drive is dying).
To fix this error if the drive is ok you need an xp install disk (doesn't matter whcih version, borrow one if necessary) to get into recovery console.
You need to boot the machine from the xp Cd (so boot order must be optical drive first - can set in bios, sometimes also key for one off selection). When it gets to screen to set up windows, hit the R key - this will start recovery console (it may ask for Administrator password, which is usually just blank - but usually doesn't when you have this error message.
At the command prompt type chkdsk c: /p and hit enter. When this has completed (should say it found errors), type fixmbr and hit enter and then Y when it asks. Then type fixboot and hit enter. Now type exit (will restart machine), and boot normally - hopefully your installation will be back up.