Yes, you need to know what hardware you machine has, in order to know whether you can put Win 7 on it. These requirements are for the lowest amount of acceptable operation.
These requirements are (as I have tested them):
A CPU of 1.5 Ghz or better. You find this out by right-clicking on My Computer, going the list and clicking on Properties. This will also tell you how much RAM you have.
1 GB of RAM. There doesnt seem to be appreciable lag in opening programs with this amount of RAM installed. At least half of you have this amount already; the rest will need to spend the $40 to meet this requirement.
A 20 GB hard drive or larger. Since even the cheapest computer these days comes with at least 40 GB or more, you should be fine without even looking.
A video card that is DirectX 9.0 hardware compliant. If you have a installed a video card at any time in the last three years, you are OK. Dont know if you have a video card? Look at the back of your tower; if the blue cable is coming from near the bottom, you do. If the cable is coming from the top(or top of the middle area), you dont. But thats OK, video cards are very inexpensive (unless you intend to play video games or start editing movies).
A DVD drive. Also very cheap.
There you have it; the least you need to run Windows 7 on your existing computer.
Next time, can you just put Windows 7 in the computer and say “Go!”?