Whats on the old 20G drive?
Is it your old version of XP pro?
If so just format it and drag the data from G to it and then check its all there before doing any thing else.
TAke a a new image of the XP pro partitionand verify it before proceding further. This is in case the next part fails
Then use a third party program (partition magic or paragon )to merge the partitions on the 40G drive.
If everything goes well then you will have your opsytem on the 40G drive and data on te 20G drive.