Inquiry about Galaxy Tab 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.