Malware nightmare

My antivirus program subscription expired a month ago, leaving my computer vulnerable to viruses. Last week a number of unauthorized purchases (Prepaid phone cards) were charged to my debit card. I immediately contacted my bank and renewed my subscription to Avast. I did a full scan, finding two threats; Win32:Adware-gen(Adw) and Win 32:iBryte-AH(pup). In addition, when I click on the Firefox button or internet explorer on my desktop, I'm prompted to startnow.com. The web address is changed to a search.startnow.com page, only allowing me to use their search bar. I tried using Control Panel, and selecting uninstall. I tried clicking the Firefox button, going to Add-ons, then Extensions with no results. I'm afraid to use my computer, I feel like a hacker is monitoring my PC. Please help me from this nightmare.:(

Thank you in advance,

Isabella