Win 8 problems with disk partitions

Hello!
I have had this problem for a while, which I have not been able to resolve with just googling around...

I have a 1TB HDD installed, with two partitions D:// with 200GB of space for Win7 system requirements and the rest was C:// where i store all my files. When Win8 came out, I installed it on the free space (C://) just to see how it works. So basically i had a dual boot system on two partitions of the same HDD.
One time the boot file for Win7 on the D:// partition crashed, so from that point onward, I was not able to boot to the Win7 OS.

However that would not be the problem. The thing is, that now I want to get rid of the D:// partition, and merge it with the C:// partition. This is where the problem occurs - it seems that partition D:// is still set as "System, Active, Primary partition", so I cannot format it. However, C:// is set as "Boot, Page file, Crash dump, Primary partition". As far as I understand, the system boots through D://, but as it has no system config. file, it forwards on to C:// and loads the system.

How can I fix this without performing a complete system fresh install?

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