There you go!
Make sure there is a windows key and preferably install media like DVD/CD
OK the earlier mentioned 240GB SSD seems up to the task
You might need a data cable, so order one when you buy one.
You receive a little package and inside there is a rectangular thingy
You find a spot inside the case (usually below the harddrive, that is a somewhat bigger thing that looks about the same as the SSD, just bigger in all dimensions.)
Hold the SSD ans screw it in place, SSDs are light, if it does not come with a bracket to place it in an space of an HD, you can use just 1 screw to hang it.
There is a power suply and there are some left over connectors, one with a flat plastic connector will have a J or L shaped that precisely fits one of the places with flat copper wires n the SSD, the bigger one.
The smaller one will have also the J or L shaped connection option, but exactly mirrored and smaller. This is where the data cable goes. This calbe, usually red plugs into the motherboard, you see tow others alreaddy connected (the DVD drive and HD)
Remove the data cable from your HD of that cable from the mainboard (whatever is more easy to reach.
Sometimes these cables have a little clip to be pushed before you can remove them and these will also click when inserted correctly.
Since you are going to get a preformance jump anyway, there is not direct need for a SSD, but it is a good idea.
Before you open your pc, make sure the powercord is grounded, but remove it from the pc. If you have a cat, get him/her out of the room and touch ground yourself, then open the pc up.
You have to feel comfortable opening your pc up and doing this, and also you have to not mind installing Windows, this is normally a matter of following instructions on screen, but there are a lot of people who do not like it. Also, you have to update the system and probably have to install drivers before everything works. Be advised, you probably need internet to upgrade and download drivers, so you need to have the drivers for your networkdevice ready in case Windows does not auto detect and installs it.