If you've been running Vista for some time and have installed and removed many programs, it's likely that a program settings area called the System Registry has become large and unwieldy. Some programs are not good at removing all traces of themselves at uninstall, so, over time, a lot of detritus accumulates in the Registry. This also contributes to the lack of "spunk" that Windows users notice in their PC after a while because applications as well as the operating system make frequent searches in the Registry.
The Registry becomes less efficient through the normal usage of Windows. As Microsoft has retained the Registry in Windows 7, this type of problem can be expected to continue. If your PC is performing poorly, it is likely that having too many parasitic applications and processes running at start-up, as well as Registry congestion, are to blame. Wiping everything and starting over is the solution. (And then you can think about upgrading to a 64-bit version, too.)