Before defragging:
11111111122222222222333333333333111122221111122233332221122333

After defragging:
11111111111111112222222222222222222233333333333333

Defragging organizing pieces of files from all over your hard drive into clusters where they are closer together so the hard drive has to move less to read them. Moving the arm inside the hard drive across the platters means it takes time to load things. If the arm doesnt have to move or move as far, things load faster.

If you open file 1, get done with it, then open file 2, get done with it, then reopen 1... now the second part of 1 is separate from the first part of 1, and this is known as fragmentation.

As for upgrading memory, its one of the easiest computer hardware tasks, but it does involve opening the computer case(after turning off and unplugging the computer), then opening the memory tabs on a memory slot, then pushing the new memory stick into an unused slot(preferrably next to the other stick/sticks already there), making sure that it is the same type(DDR will have a different slot size/indention place than DDR2). Then push it in with moderate force, and it snaps both tabs in the grooves to lock it... then boot the computer.

Also you want to be grounding yourself via static strap if possible while doing this. You also need to make sure whatever memory you buy is supported by your motherboard.

That's a general idea, can make it a little more specific if you want.