How do I extend the partition C:?

Since your 80gig drive is probably older and therefore closer to the end of it's life, the best bet would be to use a tool like Clonezilla, Ghost, or Acronis to re-create your working XP environment on your 380GB drive, and remove the 80Gig, put it into a drive case and use for backup.

However, Windows XP DOES support spanning a logical partition across multiple physical volumes:

Control Panel > Administrative Tools > Computer Management > Disk
Management. Right click on the existing system drive > Convert to
Dynamic Disk.

In Disk Management open the Extend Volume Wizard and add the new drive
(or part of it) to the old system drive that you just converted to a
dynamic disk.

NOTE: Once you create a spanned volume, there's no going back! You can't
just remove space from it at a later time. You can only add space.

Note: do a backup before doing any of this.

Hi

if this is 2 physical 500 gb hard drive with an 80 and 380 gb partition, then use one of these freeware utilities to extend your drive (Alway backup your system prior to doing this, use any utility mentioned in the prior post, or i found Drive imagexml, tobackup and Macrium reflect all good free tools to image your two drives)
to merge them together Symantic Partition magic is great (but its a discontinued product). you can use here are some freeware ones
Easeus
http://www.partition-tool.com/?gclid=COaKsZWn0agCFcTd4AodnXnsiQ
Paragon
http://www.paragon-software.com/home/pm-express/

those can do partition merges.
i hope that helps you.