The main purpose of the TC is to back up what's on your Mac. However, you can put other files on the TC disk and access them from the computers on your network (including Windows computers).
There are a few things to realize, though. First, since TC isn't very fast, I don't know how well it would work for iTunes and/or movies (I think it would work but I haven't tried it).
Second, Time Machine (the program) will eventually use all available space on the Time Capsule's disk. Then it will be inconvenient to add any more files to the disk. Presumably you'd have to go into TM and delete some old backups to make space. The easy way around this is to add a USB-connected external hard drive to the TC and use it for your shared files while the TM backups are directed to the TC internal disk. This is how I've got mine set up.
Third, and maybe the most important: be aware that Time Machine only backs up the files that are on your Mac's internal disk and files that are on directly-attached (USB, FW) external disks. TM does *not* back up files you "manually" place on the TC internal disk or on external USB drives connected to TCs or Airport Extremes. So, if you move your iTunes library there, you must come up with some other backup strategy.