I like Micro Center... find a good basic box, that been returned(go for the best CPU, MB inside you can get for the money with rebates and other offer...do not focus on memory, video, sound, hd space, cd, dvd, etc. that can be be added after for less...) always upgrade the total memory. Memory is normally the best aftermarket back for buck you can buy .. (look for the rebates on memory).
Get you bargain returned PC home, install you extra memory, and then fire up and first do a full restore of the OS to factory defaults to get it back to the new out the box state (after all it was a return so you want a fresh slate)
People have been trained that if their Windows box gets slow,...then it is time for a new one...and the Retailer gets rewarded....and microsoftie of course.