In my CIS 101 class, there is usually some sort of discussion of how programs get installed. Many times, whether in class or after class, I get a question that goes along these lines:
"Can I just copy the folder where Microsoft Office is to another computer?"
Now, back in the Microsoft DOS days, that would typically work, but Windows made it a little more complicated. There are a few things that are done during the installation process that you don't see. For example, some files are put in other folders on the hard drive, typically a subfolder under your Windows folder. In addition, some "services" are installed, and some information is written to a location called "the registry". So, with all that in mind, the operating system was designed to include an installation process, in part to make software piracy more difficult. Not all programs take advantage of this, but many do.
The problem with this is that when stuff is written to the registry, it doesn't always get cleaned out well when programs are uninstalled, etc. This leads to your registry being slower, as it is filled with junk that slows down your machine. A lot of people wonder why they can just reinstall the Windows operating system and see substantial improvements in speed. Part of the reason is the registry.
However, there are ways to fix this, and there is one program that does this for free, Eusing Free Registry Cleaner. This program will go through and clean up the mess left behind by sloppy uninstall programs. Definitely worth a download and a run. This is something I typically run every few months or so, and it always finds a bunch of errors it can fix. This is a program that I would recommend, especially with it being free and all. The one concern I have is the limited support for Vista and Windows 7.
Link to Download