This is what i ended up doing:
Booted a Live-cd and used Gparted to remove my XP-partition and make a new xt3-partition out of the freed-up space.
I then got the idea to use this new partition to host my homefolder. So i followed this how-to:
http://www.psychocats.net/ubuntu/separatehome
Only problem was that I forgot to change this addition "/dev/hda7 /home ext3 nodev,nosuid 0 2" to the fstab so that it pointed to my new homepartition "sda5"... Result: Unable to login after the reboot!
Had to go back and redo the fstab so that it pointed to the right place!
But in the end I got it all to work (so far at least) and I am pretty pleased with the result: Got ridd of XP and kept my tweeked Mint 4.0 the way I liked it!