Sounds as if someone has a 'backdoor' I guess it's called, 'spyware', -and is retrieving data from your computer. Maybe even a keylogger..
Get some Firewall protection, run some malware/spyware cleaners, etc. I suggest CCLEANER and SPYWAREBLASTER (both freeware, Google the titles).
First time I ran Spywareblaster, it identified about 3/4 GB of 'junk' that needed to come off, 99% of it was merely harmless junk but some was rather malevolent... like tracking cookies, offshore dialers, etc. You know, -the ones that (if you had a modem), first silence the modem prior to re-direct to some off-shore country and exorbitant connect-fee (like I have seen $49.oo per minute, etc).
The 'silencing' of the modem is the illegal part of this, as the 'noise' a dial-up modem makes during dial/re-dial would be your only clue that you are accessing a pay-per site. In the States at least, the phone company might see their way to 'forgiving' one instance of this happening to you on your phone bill and IF you agree to have a one-year block on international calling (safegards both you & them).
This happened to me once... I clicked a link and landed on a page that wasn't what I expected. It was an interestin page, so I 'lingered' for about two minutes, then, turned the computer off and went to bed...
A month later I received a phone bill with a nearly $50.oo charge for a 2-minute call to B*****os or something... After some arguing, the phone company disallaowed the charge, we agreed to place my phone on 1-year ban for international calls (if you DO need to make an international call, you have to call their Customer Service Dept. and they 'turn it back on' for 24-hours or whatever, so you can make your call in a resaonable time but they 'turn it back off' the next day...)
A few weeks later, a Federal ruling was handed down that this type of re-direct is illegal, as 'silincing the modem' is the underhanded part... they however cannot control how much a pay-per site charges (esp. off-shore, international, etc), but off topic this... In summary, if you do not have adequate protection this and other nasties can happen to you...