C Drive gone&'VIRUS ALERT' in syst tray

Hello,

I need help badly. I would appreciate in 'laymans' terms and step by step guide if possible how to resolve my problem. I have lost my C drive due to various Trojan Horse viruses that have suddenly infected my PC. I also have VIRUS ALERT permanently in my system tray right next to the time. I have ran AVG and Superantispyware, deleted the infections and emptied my Recycle Bin....I have set up in 'safe mode' and also tried to 'restore' (which only takes me back to the morning the virus was installed and no other date prior to this is available to me). I was going to format my PC but I don't have a WINDOWS XP Disc to reinstall everything....I have scanned & scanned & got rid of everything to do with Trojan Horses but I am now at the end of my options....it's either ring a PC expert to do it for me (which I am sure I could do if guided) but I don't know how much this would cost or buy a new PC. Could someone please help, I would be very grateful.