Comparing Google's Desktop search with Windows Vista desktop search. Accessing the Google Desktop search is easy, the fastest way is probably by pressing twice on Ctrl or just F3 as it seems like it'll replace Vista's search hot-key. But even if you turn off or had Google Desktop turned off it seems like it'll start-up again by pressing F3. So you won't get access of Vista's search by that Hot key. In Vista when searching, the fastest way except then from pressing F3 is by clicking at the start button and use the limited search space in the menu or opening Windows Explorer. Another way is to access the Search with Google desktop is in the sidebar or in the task bar/floating desktop depending on how you have configured it. But there are no desktop search function provided neither in the task bar or desktop from Vista. Doing a conclusion, Google wins here.
How about the performance changes when they are indexing? We know that Google Desktop starts it's indexing when the computer is idle. With Google Desktop there is at least to say noticeable performance changes when the the indexing has started, fortunately you wont notice it as you are not using your computer but this is not the case with Windows Vista search indexing. Windows Vista search indexing will run continuously and the performance will decrease much. This performance decrease will lower in the time as less files need to be indexed. Even thou both of them use's a lot of resources when indexing Google desktop is a bit smarter to not index while the computer is in use.