Playing with testingCheatsEnabled true for more than short periods can really put a strain on things.
But it does also generate error logs within your game which looking at can help provide clues.
Personally, would definitely NOT recommend putting that cheat, set to true, into any userstartup.cheat file.
That said, you have either a bad file, a file which hasn't been updated to compatibility with whatever your highest number EP is, or a conflicting file somewhere in your Downloads.
More than likely, something which was incidentally included with a lot you downloaded is the culprit. Custom lots too often include many things besides the building(s), decorations, terrain and furnishings. Using Clean Installer to install custom lots allows you to choose which parts of the lot file you actually want to have in your game.
First thing I'd check are hacks -- especially (based on your description) any mailbox global mods. That's just a guess, but does seem to be a common bugaboo that causes the same symptoms.
Run Clean Installer -- it will highlight all your hacks in the Downloads folder in red . Look for any with the word mailbox in them or which start with the word zMailbox.
Temporarily take those out of your game and see if the problem is resolved.
If that doesn't set things right, then the tried-and-true 50% method is the surest way to isolate the culprit(s).
There is a smaller probability that the problem lies with a custom Sim you may have installed and which is in your Saved Sims folder -- but the greatest likelihood by far is conflicting content in your Downloads folder.
NOTE: It is also a possibility that you find the problem lies with a Custom Career or Custom Major (if you have any of those installed). If that is the case, it is vital to have any Sims who have employment in that career quit their job first, before deleting the problem Custom Career from your game.
Oh, and do install the patches -- their purpose is to fix bugs and problems which can develop from those bugs which the originally shipped versions of the game contained.
Info on the 50% method:
http://www.sims2wiki.info/wiki.php?title=Game_Help:Finding_Problem_Custom_Content#The_50.25_MethodLink to Clean Installer download:
http://sims2pack.modthesims2.com/EDIT: One other question: Is your new Vista machine a dual core computer? IIf so, you have to set the Sims 2 affinity for single core, as the game itself was never intended to work on dual or quad core systems unless the affinity iis changed.
