There are various forums with tutorials about making skins and phone modding. I assume you already have Mobile Phone Tools:
A couple of the ones I visit:
Avast. Works with both, don't notice it on the background, is not a resource hog nor is intrusive.
Has stopped everything so far that tried to get in my computer. Use it with both Windows XP and Vista 32-bit.
Keep in mind that although electronics is advancing there is just so much you can cram inside the (already crowded) space of a mobile system, much less an ultraportable.
I think the wisest is to get one with a single battery that gives...