Windows 7 with a 64 bit version is what you will generally get with a new PC. It is different from what you are used to but it is the future of Windows. XP will be getting less and less support in the future.
Now we will get to what you need to know that gives you the most bang for the buck.
How much memory do you need.
I think 2 gigabytes is the minimum you will need. Many units come with 4 gb. If possible get 4 or more gigabytes
How big a hardrive?
For average use a 250 gb drive is fine. Have some music files? Then 500 gb might be better. Video or your favorite TV shows? 500 gb or more depending on how many hours of video you are going to store. The cost of a hardrive is pretty cheap.
How much processor power?
Much depends on what you use your PC for.
Is your primary use is email and surfing the web – then a dual core or better processor is going to be adequate for now. Most new processors are dual core or 2 processors on one chip with quad core available. Speed ratings are becoming pointless except to compare speed within the processor line itself, like core duo or x2. Email me if you need a further explanation.
If you are a heavy duty game player, use a lot of video applications, or just have a need for speed, then you are in the quad core category. You know who you are. Unless you need to have cutting edge equipment and are willing to live with the problems of having the latest you can save a lot of money by building what you need.
So, Do think you might be ready for ...
One last thing to think about is buying a Apple Macintosh or Windows based machine.
A Mac is slightly more expensive than a Windows based machine but is much easier to use, rarely gets a virus and comes with most things you need. If most people used Mac's I would not repair many computers. Also Mac's have switched to Intel processors which means that you can run a copy of windows on a Mac. This was never possible before. With software you can now run Windows from inside a Mac which will really make things interesting.
What this comes down to is, unless you use a special windows only program on your computer you should look at a Mac to see if it is you.
I do not use Mac's or Windows for myself. I use Linux which is a free software system that appeals to power users and people who like to tinker with computers. It is becoming more mainstream and could save you quite a bit of money. Ubuntu is a very easy to use Linux distribution that is free. I only use Windows for a few minor things having to do with my repair business.