Between your post and your heading, I assume you have a HDD with one Partition that you boot from (C that seems very full, plus another Partition you consider expendable. So I assume you can back up the data on the second Partition, or just delete it, leaving that Partition full of NOTHING useful.
You can Delete a Partition, and you can Expand a Partition to include empty space on a HDD unit (as long as it is contiguous - that is, immediately after the active Partition). The problem you face is that Windows will NOT Expand a Boot Partition. For that you need someone else's software utility. Acronis Disk Director is one, but you must pay for it. There are freeware tools also, but I've never used them and cannot tell you how good they are. Here's a forum post with a recommendation of that sort of tool.
GENERAL CAUTION: Any work like this definitely carries the risk of losing ALL your data if anything goes wrong. Before starting, MAKE A BACKUP, then VERIFY that the backup is good. If you're really cautious, make TWO backups!