I like to make a separate partition for Windows (C: ), one for applications (D: ), one for the everyday user files (E: ) and one large everything else partition (F: ). There's really not a good argument for separating the Windows and app files though. Even if you do have a separate applications folder, some applications like Microsoft Office install large portions of the code in C: (common/shared apps). On the other hand, I have gotten into the situation a couple times where I was forced to reformat and reinstall the OS, so having your user files separate from your C: drive can be very advantageous. I also use Microsoft TweakUI control panel to move My Documents from the C:\Documents and Settings default to the user files area. That's a life saver if you find yourself having to reformat the C drive. As for the size of the C drive, I make mine about 15 GB for the OS alone. Along with the OS, Documents and Settings (the temporary internet files can become pretty large), swap files, and hibernation files (if you enable hibernation) reside there. Also, every Windows Update backup files (those that start with $NtUninstall and are used to restore the files if an update needs to be backed out) can get pretty large as the years roll by and they don't go anywhere. If I were going to have a combined OS and Applications drive, 30 GB is probably a good number.
Since my "real big" area is for games, video and the like, I like keeping it separate from my user files. First, if the "big" partition fills up, it doesn't affect my ability to fetch email, edit documents and the like. Secondly, since the user files are the ones that would really hurt if I lost them, I can quickly set up a backup to a DVD-RW or two since it's in its own smaller partition. I still find it amazing that all my really important stuff created over years usually fits easily on a single DVD. I keep my user area to about 5 GB (per user).
For your specific application, you might want a combined OS and app partition (C: ), a user partition (think of it for correspondence and email [assuming you move your email files to that partition]) (D: ), a work partition that's pretty good sized - I'd guess 50 GB or so - for all your web-related files (E: ), and the remainder for games and videos (around 240 GB) (F: )
For partitioning, I use PartitionMagic, which I've used for years. I'm a bit concerned though that it hasn't been updated in a couple years. Still, it works, so maybe it just hasn't required it. I find it very easy to use, and it's never screwed up a disk yet.
