Once you use your boot disk and get a shell prompt - make sure you can get to your linux hard drive.. type ' mount ' and you should see a list of your mounts and the drive's they are on. Look for something like
/dev/hda1 on / type ext2 (rw)
The hda1 may actually say hdb1 or something like that - but the " on / " is what is important.
If you can get to your linux partitions then you're in good shape. From there type:
If that doesn't return anything, it should be located in your /sbin directory.. just type /sbin/grub (it might be /usr/sbin, can't remember, I still use lilo)
Once you are in grub, you'll have a grub> command line. From there type:
this should reinstall grub onto your MBR so you can dual boot again.
Be sure your /etc/grub.conf file is setup the way you want it, before you do this. Some config info for this is at http://www.gnu.org/manual/grub-0.90.
And the reason I'm pointing you to there, is this can be a long an involved process. If you have specific questions after reviewing it, please feel free to ask.