Partition Issues- In preparation for Windows 10 from 8.1

Hi all, I've been stalking your forum for what seems like years but now I finally have an issue which is stumping me. I'm not a wizard at computing, but I'm not an amatuer either so appologies for any issues that may arise!
I currently Have Windows 8.1 Pro (Upgrade from Win 7) installed.
In preparation for Windows 10 I was having a clear out before deciding a complete format would probably be best to get rid of the years of rubbish accumulated (From Vista days).
My problem is this, at some point since owning Vista on the computer an additional partition was created which I never use as it's nearly always full and has nothing of use on it. Unfortunately this is where I get confused as both partitions seem to do different jobs (System, page file etc)
Is there any way to clean it up so everything is on one nice partition for Windows 10?
I'm unable to delete or merge in Disk management I think due to them having the different functions spread over the two?
I seem to recall it's ended up like this when I upgraded from 7 to 8 so I could dual boot (Back in the days when Win 8 was an unknown entity)
For reference I had the full version of Vista, then upgrade versions of 7 and 8.
Many thanks in advance,
Shaun