First of all, sounds like you have only 1 actual hard drive split into 2 partitions.
Do this for me, Start button then in the run box type (at least part of) Computer Management then select it from the search results, once it appears under the Storage subheading on the left column, select Disk Management, there it will show you how many hard drives you have and how they are divided + any other storage drives.
Question #1 YES! You can point Windows to D: for temp files and browser cache also, I would make a D:\TEMP for this purpose.
Plus lots of games and applications can be copied over to say D:\Games and D:\Apps directories and tried, for the ones that work make new icons and delete the old ones on C:
*Note, before testing on D: move the original ones to another directory also (on C
, to avoid having false positives of it working on D: [Registry related...]
Question #2. Yes
Question #3. Yes, besides temp files and browser cache, Virtual Memory (pagefile.sys) and a Hibernation mode file can eat a lot of space.
More details will be forthcoming, wanted to get your quick answer out first. 8)