That is excellent.
Ok once it comes run this scanner so we can see which RAM is compatable.:
RAM Memory Upgrade: Dell, Mac, Apple, HP, Compaq. USB drives, flash cards, SSD at Crucial.com
Then we can sort you out with some RAM.
To fit a new hard drive is usaly disconnect two internal cables undo a few screws or clips.Remove old hard drive and fit new one.
We can help you out with reinstalling windows it is not that hard.
You should give the tower internals a good clean while powered off.
We will get you to give it a service via instructions and get it up and running smooth for you.
I think £40-00 is an excellent price a lot cheaper than replacing a mainboard,CPU and possibly a power supply.