Unmountable_Boot_Volume isn't the type of error that typically can be resolved thru safe mode...
The BSOD probably looks like this:
STOP 0x000000ED (0xaaaaaaaa,0xbbbbbbbb,0xcccccccc,0xdddddddd) UNMOUNTABLE_BOOT_VOLUME
What does the 0xbbbbbbbb look like? If its 0xC0000032 then you're not gonna be too happy - it translates to not being able to read the file system... If you have an XP CD handy you can boot to the recovery console & run 'chkdsk /r' on the drive & hopefully it will find/fix whatever is causing Windows to not be able to mount the drive.
I'd suggest googling the 0xbbbbbbbb value (in its entirety) along with Unmountable_boot_volume if it doesn't match what I put above.
good luck!