Increasing age is not relevant for Windows, an operating system it is not a human. The number of installed programs, no matter. The problem arises only when too many programs start automatically at startup ("autostart").
The real burden: The modern processors are very compatible to make process very easy, but every autostart consume additional resources. This causes, when it starts, no more memory available, Windows stores nonessential data on the hard disk, which affects on the performance and this leading to the system collapse as well.
I think, additional activities on the hard disk delay the onset of new applications, even if later they are working as fast as usual. The solution: Turn off all the "autostart" and unused services. The easiest way to do this is through the "System Configuration Utility" msconfig.exe. In Windows XP you have to use "Run" and Windows 7 or Vista "Start Search" menu "Start."
In addition, on the general tab you can select only "early" Windows needs. If this fails something important to the next time Windows starts, just put the check back to its original state. In the tab "Services" it is essential to activate "Hide all Microsoft services" to avoid the elimination of a service that is very important to Windows.