I have a 160 gigabyte hitachi deskstar hard-drive. i have a partition with windows Xp and one with slackware 13 a dual boot using lilo. however windows only detects my hard-drive to be 131 gigabytes this is what my bios thinks as well.the undetected space is at the end of my hard-drive. Linux sees the whole thing.
I tried to install linux on this undetected 29 gigs once and it rendered my computer unable to boot. I tried to update my bios to see it and that screwed my computer up so bad i had to flash my bios so i am somewhat opposed to bios updates.I ended having to format my comp and reinstall everything without touching that space.
I want to use cfdisk to format this area to Xfs, just to use as free space.My root linux partition is ext3. I want to know if i risk ruining my computer again. Would making a logical or extended partition be less risky than a primary. i am not even sure if cfdisk can write on this area yet. I am pretty new to linux i have used linux computers before, but have only had it installed for a week or so.

It sounds like you are using an IDE drive with the BIOS set to the older CHS mode. Edit the BIOS to change the disk mode to LBA.

Read more about it in this thread.