One of the main reasons why your Ultiamte install goes slow after 2 weeks is cause you use a pirated version.
A fresh clean install of Vista Ultimate with a legit ISO wouldnt do that. The pirated ones all have something wrong with them.
Believe me i know since i got my legit version now. I noticed that Vista Ultiamte would take more RAM, HDD space, and lag after running it for just a couple of days. This was using a pirated version with a BIOS Crack. But after getting my MSDN and a real M$ ISO of Vista i found that it doesnt act anything like it. I ran it for over 2 weeks without any lag or aything.