So your hard drive has two partitions, C and D.
You want to increase the size of C drive 20 or 30 GB.
Backup, save, hide it someplace safe all the files/info on both C and D that you don't want to loose.
Run> disk management> delete the D partition > increase C >initialize and format what is left to the smaller size D.
Restart with your installation disk and re-install.
Copy the saved, backed up or hidden someplace safe files back to where you want them and your a spring chicken all over again.
You will fell a little older after installing ALL 5 billion or so updates that have been implemented since whatever version of your install media though. . . ;>(
A lot of time and effort for 20 or 30 GB . . . ;>)