Why is my C: drive filling up even though I install programs on D:?

I've heard that Dell computers use drive D:\ as the default for Program Files, etc.

I have HP, that defaults to C:\, and a train simulator that defaults to my drive C:\Program Files(as most installations do)...but it must access the Internet, so to bypass the Windows Protective Services, I install it to C:\TS2009, and allow the .exe files to access the Internet through my firewall & Internet Security software.
This has eliminated a ton of problems experienced by other users of this game.
But I can't figure why people want to change locations for TEMP, etc, because if trouble happens, the files are spread out all over the place & seems that would be harder to diag.

I take the view that anything that relates to the 'operating system' (including links with the registry, shared / common components, etc# goes on the OS (C:) drive, as it all needs to be backed up (and restored) together. I have a separate (logical) drive for my data files. I do keep portable applications (= nothing needed but the contents of the program folder; no install required) away from the C drive. As you can see on some of Leo's other answers on copying your software to another PC, you can do nothing with the majority of the contents of the program files folder - it won't work without the matching registry entries, etc. So treat it all as a single entity, and hold it all on a single drive.

I have been "optimizing" computer systems for many years. I've always run my system with a C: boot drive and a physically different D: data drive. Most programs can be installed to run from D: But only a few will require to be installed from/to the C: boot drive. I've run my HTPC for 6 years straight 24/7 only needing to shut it down for hardware upgrades and to replaced a couple failed components.

Along with the aforementioned tips by others, there are some more things you can do to keep your C: drive clean.

You can move your swap file over to the D: drive.
Set up a normal procedure for "cleaning" the junk files. Just be careful what you delete. There are utilities such as "CCleaner", and "Advanced System Care" that are for the most part conservative in their settings.

The BIGGEST drive space usage comes about if you are apt to install and remove a lot of applications. Most of the time, each new install will create a "restore point". And guess what? Unless you actually purge those restore points you will have dozens (or more) copies .. each one using many GB of drive space. For my system, I use a Vertex 30 GB SSD. My C: drive is never less than half full. (about 12 to 15 GB used out of 30). My D: drive is a fast SATA 120 GB.

I'm running a Win 7 Pro 64-bit OS with a 4-core 2gb processor that runs 45 wts.

Use Advanced System Car to clean your registry and get rid of junk files. That and CCleaner are freeware apps. For a few bucks more you can register the "pro" version of Adv. Sys. Care that will give you some more options. But the freeware aps will usually be all you need.

When you uninstall a program, after doing a system scan for junk files, also use the Registry Clean option available in CCleaner or ASC.

Next right click on your C: boot drive and bring up it's "properties". Click on the "Disk Cleanup" button. On the next window that pops up, click on "More Options". In the "System Restore and Shadow Copies" box, click on the "Clean up" button. Read the info there. That will delete all BUT the most recent restore point. In most cases that can get you back several GB of space.

Just be sure everything is working okay and you have current backups. The only downside to deleting all but the last restore point is that you won't be able to restore back 2 or more copies. I've never had to use any more than the most recent. But then I frequently do full and incremental backups. Of course YMMV as they say.

One benefit I've always experienced by using a minimal boot drive space is that it allows me to take advantage of the BIG performance increase by using a fast SSD. And keeping that drive small allows you to take of the falling prices for the smaller SSDs.

I noticed my C:/ drive filling up rapidly also. I hadn't installed any software, so I was puzzled, until I was in Windows Media center and noticed that I had accidentally turned on the video recorder for a scheduled recording every week. It is very easy to do, especially by novices like me. I was only trying to learn to use Win 7 Media Center and just clicked the wrong buttons.

You forgot the best tool from Microsoft, for free, at: http://onecare.live.com/site/en-us/default.htm?redir=true
Choosing either Protection, Clean up or Tune up. Clean up will clean the registry and delete all temporary files. Protection will clean the attached hard drives from all known MALWARE at that time. Tune up is a defragmenter program.

In addition to Rick's message : In noticed that the winsxs folder (vista/7) can grow out of proportion (10 gb in my case!); this is where shared and/or old dll's are stored. Better left alone; discspace is cheap.