Download the GParted LiveCD or use the Ubuntu CD. Default account is "user", with password "live" for Gparted LiveCD. There is no root password, so if you need root privileges, login as "user", then run "sudo" to get root priviliges.
Grab your linux partition at the start, drag it to the right until its the size you want it. Hit apply.
Backup in advance as a safety net.http://en.wikipedia.org/wiki/List_of_disk_partitioning_software