Can I install any of the software that we can install in an desktop PC? Can I use Adobe Photoshop 3, Adobe Photoshop 5? Can printers be connected through USB to this? Does it support any external USB mouse.? Can I insert a pen drive in it.
The Galaxy Tab runs the Android OS, so no, you won't be able to install any software that is made for a PC (unless they have re-made it for Android).
I do not believe you can connect a printer to it, but there may be ways to print wirelessly (I have no experience with this).
No mouse support that I know of as it is a touch screen device.
In addition to Bryan's answers - the very last question - No, you can't connect a USB flash drive.
For file interchange issues you may use Dropbox. This utility is available for MS-Windows, Mac OS X and Linux (and even iPhone); you will get a folder, which is synchronized between any of your machines. It's very comfortable, you can organize your shared file set on a desktop machine, then you have just wait for a while to appear it on your Android device. I think it's more comfortable than using a pendrvie. (Dropbox is a commercial service, but you get 2 Gbyte space for free.)
For iPhone and iPad, which devices have no SD card slot nor any other way to exchange files with computers, it's a life saving solution.
Here is a review that should clear up all of your questions. Except whether or not it would run Photoshop (ANY version) and the simple answer is no, it will not run any windows or Mac software. I don't even think there is an emulator, but I may be wrong. Either way it doesn't have the hardware to run both an emulator and Photoshop, it would probably not run Photoshop effectively even if there was a native version. There is also no USB port of any kind, not even micro; it charges through a Samsung proprietary cable.