Please inform me whether you are getting a screen like this:




If so, be assured, the 'FBI' notice is actually quite fake, first of all.

Second, how long ago did this infection happen? or how recently can you be sure that your computer was still working?

Third, can you restart in Safe Mode? try shutting down, or shutting Off, your computer. Then from a power-down condition, turn the computer On, and start pressing F8 key at the top of your keyboard, once a second, until the Safe Mode choice screen (all text, black-and-white, with a white selection bar on one of the menu choices... ) appears.
Then use the up or down arrow keys to move the selection bar to 'Safe Mode'... can you get this far? if so, stop there and reply to me, please. I will guide you further through the process... or actually, you might just choose 'Repair my computer', then when
System Restore
comes up as an option, choose to restore your system to a previous date prior to the first appearance of the MoneyPak screen, and a date when you are quite sure you remember your computer being fully in working condition.

That ought to get you at least temporarily clear of the problem. Then when you have a normal-looking bootup, immediately, before anything else, fully update whatever anti-virus software you have, then do a Full Scan or a Deep Scan of your entire hard disk (or your entire C: Hard Drive, at least) to clean off whatever sneaked into your system and produced the bogus FBI screen.
And, if you don't have a good anti-virus, then please try AVG Anti-Virus Free 2012:
http://free.avg.com/us-en/homepage
and once it is installed, update it, then do a Full Scan or Deep Scan. Hopefully that way, your problem will not recur.

I hope this provides you with the answer you need to your question in every respect. If so, please rate my Answer, or else reply informing me what rating you wish applied. Thanks.
(If any other info is needed to make the answer complete, please reply and I will try to provide it.)