You'd only need something like Photoshop or PaintShop Pro if you wanted to do editing.
If you're looking to do simple, unedited screenshots, then just hit print screen and then paste it into MS Paint. (If you want to upload here, you'll need to convert the shot to .jpg, so go to
www.irfanview.com and download that nifty tool for free.)
There is also a free demo of PSP at JASC.com. It's great for screenshots. (There are freeware alternatives to them as well, but I use PSP so I don't know them by name.)
If you decide to use a screen capture tool like GrabClipSave, then your shot will automatically be captured in .jpg, eliminating the need for a converter like Irfanview.