Windows Folder Too Big

Hi, I have two computers. For clearer understanding lets call these 2 computers computer A and computer B. A few days ago, I got a low disk warning notice stating that I should cleanup some of the older files that are not being used on computer A. I first ran the cleaner manager that Windows XP Home provides, and deleted some of the files in there. I then ran another tool that is included with McAfee Security Center. The tool is called QuickClean which cleans the junk files that windows creates such as cleaning the temporary internet files, windows temp folder, cookies, registry, and trash etc. After doing so, I did gain about 1GB. However, this only brought me to 1.7 GB of free hard drive space. So I then browsed through my C drive, and right away I noticed that the windows Folder was up to 4.3 GB. Now I do not recall what the Windows folder size was originally with computer A, but when I compared computer (a) to my other computer, that is computer B, there is still a difference of approx. 2.7 GB of used file space between the two computers. So in other words, computer A's windows folder size is 4.3 GB and computer B's windows folder size is 1.6 GB. I can understand that not 2 computers are alike, but what I do not understand is how this is a huge difference. In addition, once in a blue while, Microsoft Word starts to load by itself and then hides itself. Meaning I did not ask to open the program and after it loads there is nothing in the taskbar, and no processes of Word are being showed in the Task Manager. So far this has happened 2 times one in each day. Has any body seen this happen before, and what would you suggest to do about the windows folder taking up so much hard drive space?

To make things clearer about the windows folder problem, I am looking for something that I could based on that is a windows needed file. Once I could find a way of basing that it is a windows needed file, then I could delete the files that are not based on. So far what I have found was if the file was created in the time period of December 2001, it appears to be a windows needed file. However, this sounds very dangerous just to rely on that date. Is there anything else that I could base these files on or is this just too dangerous to rely on?