I'm not totally sure but you may have to go through the whole reinstalling process if the data on the 2nd partition is more than the target you are trying to acquire. You may however be able to transfer the data from the 2nd partition onto another HDD, use a formatting tool (maybe Acronis or Partition Magic) and merge/split the current HDD accordingly.
PS: I'm not totally sure so you could wait around for confirmation. I used to play around with Partition Magic but can't remember all it's functions though.
Edit: Seems Partition Magic is no longer around and it did not support Win Vista and 7, Acronis Disk Director
can do the trick though.
Partition Manager allows you to resize, move, copy, split, and merge partitions without losing your data.