BTY:
1) you have to have two open bleeding cuts bleeding profusely for there any risk. That isn't you.
2) Microcuts - bogus scare tactic. maybe holds some water if you are talking about the anus or rectum, but not on the hands
3) in 40 million cases of HIV infection, no one has probably ever caught HIV this way
You will not become HIV + on account of this event. Under the finger nail is skin, not mucous membrane. The person touched you with blood? All of this is occurring outisde of the body and HIV does not live well when exposed to changes in temp, light, ph.
Furthermore, what possible reason do you have to believe this person has the disease. Forget this and move on.
I personally don't think you have a high risk of contracting it, however, I would talk to your doctor and explain what happened and get his/her advice on what to do.