There are many purported reasons for so-called "WinRot," ranging from registry corruption to "DLL Hell" to disk fragmentation, and there are plenty of entertaining rants about WinRot available for your perusal.

One commonly accepted cause for WinRot is "DLL Hell"—a term referring to the myriad problems of managing DLL libraries. Multiple versions of the same DLL can cause any number of problems in Windows, from crashes to random performance problems. Related, and sometimes confused with this, are problems relating to uninstalled software, drivers, and utilities that can haunt a computer. Have you ever tried uninstalling a Norton product? That'll bring you WinRot in no time.

Microsoft is hoping to rid itself of rot with Windows Vista, although one wonders if their proposed solutions will do much good. Windows Vista will automatically defragment hard drives, use a new feature called SuperFetch to load frequently used programs into memory before they are needed, and utilize a new control panel applet to help identify specific areas that are causing performance problems. Furthermore, .NET and WinFX are being designed with DLL Hell in mind—something they have been waiting for since .NET was released.
http://arstechnica.com/news.ars/post/20050808-5181.html
Windows Vista to end the need for reinstalling due to "WinRot?"

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