I had a woman, my nominal boss, who was like that but as irritating as she was, I felt a little sorry for her (because I had the opinion she couldn't get past me :-) We had software that I could check and which I understood better than she did so that she could try to do ugly things (usually to cover up her own mistakes) but which would backfire (why I felt sorry for her, she couldn't "win" ultimately). The other women knew she was like this (she'd been working for the company since the beginning of time and I was the newest member) and were afraid of her abilities, her lying and blaming others to cover her own posterior and I was glad to know and made sure I was enough intimidating back to her that she rarely bothered me.

I would target the not-very-smart person and just let her see you doing well/better than she can enough so she gets a little afraid; I'd have a talk with the IT person and make friends with them so they don't make the mistake again of letting others have access to your machine and/or I'd make friends with their boss :-) so they too are not likely to mess with you? A little show of competence, without aggression/bragging or anything, can go a long way so people know you're your own person, are competent and don't take kindly to being messed with. That you know yourself and stand up for yourself and your abilities, the "baddies" will pick other people who might look easier for them. Just show you are "stronger" than they are, have a boss praise you in public or say something impressive in public, etc.