Windows 7 Disk Manager with partitions on 500GB Hard Drive

Hi I dont understand a whole lot about using Disk Manager on Windows 7, plus I dont want to lose any data or cause any kind of file corruption. I was wondering a safe way to merge partitions. But here is the thing with my partitions as it is, I have in order: Primary Partion with 25 GB space free, then beside that by C:/ Partition with my OS with 186.3 GB space on it then my D:/ Partition with 254.45 GB space. I was wondering how to merge all the partitions into my C:/ partition without any loss of data. I do have an external hard drive to back up any partitions if need be. Just looking some instruction and help since I cant see to locate much info on my particular situation on Google. All the partitions are on one physical Hard Drive.

Thanks,

Bobbby103

bump