There isnt really anything you can do by profile to speed up loading. Any tweeks you do would be to the OS itself which would effect all profiles. Profile tweeking helps a lot if your loging on via a roaming profile as you have to load that from a server so smaller is better but it wont do much if anything loging on locally.
As far as spyware and stuff is concerned it depends on what gets downloaded. A lot of it sits in the profile of who downloaded it so wont effect anyone else. Some of it tho can work in anyones profile and could effect you. Best way to stop this is just to make his account a user account then he wont have permission to load programs or write to your profile. Its actually a pretty good idea to give yourself just a user account as well to stop stuff like this but it can be a pain in the ***. User accounts are a good way to stop some viruses too. Obviously you have an admin account on there as well for loading stuff or fixing when need be.