I would strongly discourage you merging your two partitions. Usually, you want to have all your software on the C drive and all your files on the D drive. If you have filled up your C drive with ONLY software, you may INCREASE the partition by shuffling some empty space from D to C. Surely you can download a time trial partition manager that allows you to do this before it expires.
A reminder on the importance of this came just last week, when a friend of mine got her machine crashed, and lost all her work (3 months) that was saved on C. Unfortunately, she found some very bad IT advisors - the same that crashed the drive - who reinstalled XP and all her software - overwriting what may have been recovered.
To make the disaster complete, they then proceeded to save the few recovered files also on C. If you have a D partition, your files are safe during system crashes and reinstalls. Extra backups never hurt anyone, though