if you are paying someone to fix this, ask them to
1 - install an operating system that supports users and rights. XP is fine if you like windows - pro is even better.
Get them to setup an administrator account, and make it very random - they can write it down and you keep it.
2 - setup your account, your wifes, and the girls accounts as simple users - even better guests.
3 - get them to give you an image of the final build - on a CD / DVD is fine. Norton Ghost 2003 will do this nicely.
4 - Install anti virus software. Avast is good and free for home use. Set it to auto update every day (defintions and program).
Dont believe that changing browser or platform is the answer.
Firefox has more serious exploits than internet explorer right now, and Mac's security through obscurity is hardly better than any security at all.
Use the security offered by the operating system to ensure that you control the software on the machine. If your users cannot install stuff - then malware will not be able to install itself either.