One more thing to consider is Windows 8 will be releasing this year (supposedly anyway but all markers point to it).
Next thing to consider are the drivers. A lot of the premade computers come with the drivers already installed and a lot of them do not install from Windows Update so you have to get them manually from the manufacturer's website. This can be a little intimidating for a person doing this for the first time but people here should be able to help if you run into any problems.
I will disagree a little with Apoptosis and tell you to check your Vista to see if it is 32 or 64 bit. If it is 64 bit, then get 64 bit windows 7 but if it is 32 bit, do some research to make sure all the touch functions on your screen will work in 64 bit.
Installing Windows is the very simple as it guides you through the whole process so do not fear that part
If by weird chance you are around South Central PA, I will be more than glad to help you out in person.