Is this a partitioned drive or 2 separate drives? You can get software to combine the 2 partitions if it's on a single drive, or you can get a new drive and use a cloning software to copy your current drive to the new one.
You can move your My Documents folder over to D: (right-click My Documents and pick a new location), make sure your recycle bin is empty, right-click on the recycle bin and change the % of space it uses to less. Change temp I.E. files used to a lower amount (it's usually set pretty higher than needed, even 100mb is enough). Check Add/Remove programs for anything you installed that you don't use. If you can also safely delete any files in C:\Windows that start with $Unisntallxxxxxx that are in blue. Those are files kept by various updates incase you need to uninstall them. That is very rarely needed, only in case there are system issues after an upgrade. If your system is running fine as is aside from the space issue, you can delete those files. Make sure you only delete those files though, and not others in the folder.
I would get a larger drive. Clearing out space on a totally full drive, unless your My Documents folder is very large and moving it over to D: helps, is a very temporary solution. Do some looking into cloning software, there are a few free ones, and a new hard drive will sometimes come with such a utility to help you upgrade.
In addition to having more space, a new drive tends to be a safe move as small drive usually means older drive, which in turn means greater chance of it failing over time than a new one. Not that a new drive is not capable of crashing, but the chances are hugely smaller.
Way back in the early 70's I was a Field engineer on CDC large systems. We had a big( taller than me at 6'6" and 20 ft long). It did paralle head reads. All heads turned on at one time. In its day it was fast. Thats were the operating system was and everything else on serial read drive.
I thought that was what I was doing with my SSd drive.
Ah, SSD, so it's pretty new to begin with. If you got an older one, it will be a bit constricting. You did not say it was a SSD, and these days when you run out of space it sounds like the drive is a 20 or 40 gig old one. Try to move My Documents out of the default location to the other drive, and you should be able to chance the setup location for most programs, although you'd loose some speed if you move them off the SSD.