The two ways to do it:
(1) Buy a third-party program like Partition Magic. There's shareware with the same functions also.
(2) Make an 'image' using a program like Acronis TrueImage to another drive. From the XP CD, reinstall XP. Tell the partition manager in the XP setup to delete the old partition and create a new 80 Gb one. Then use True Image again to restore the image on this now bigger c:\-drive.
I recently installed a new external hard drive to free up some space on my 80 GB C drive. The C was previously divided into two logical drives, C and D with 40/40 partitions. I tried to restore the C to its original 80 GB size by removing the logical D drive in Disk Management. However, Windows still only sees the C drive at its previous size. How do I add the remaining 40 GB to it so that the 80 GB are contiguous on the same drive?