If Panda Antivirus, presumably the online scan version is detecting viruses, the Panda site will give guidance on how to rid your self of these. Often there will be a download tool available to rid yourself of many of the more common issues.
You should have permanent antivirus on your computer and not rely on online scanners. AVG are a good freebie, but there's no substitute, if you have the money, for a good brand, paid for, antiviruse solution that has frequent updates. On the subject of Panda, we use this at works and with the aid of the frequent automatic updates, have not had a virus cause problems for over 3 years. Not bad in the current ever changin virus environment.
If your PC is this badly infected, since you have no effective antivirus, it is probable that you also have spyware and adware taking over your computer. These are an additional cause of problems on top of viruses. Try downloading adaware from
http://www.lavasoftusa.com/ a freebie which works well and should talk you throgh it.
Antivirus and anti spy ware / adware software is only as good as the latest update. You need ot ensure your computer is set to subscribe / update frequently, preferabley daily for antivirus.
As you have so many problems, I also anticipate that you your M$ Windows, whichever version, is not fully patched. M$ software has a number of holes in it which the unscrupulous exploit with viruses and stuff. Make sure you have Windows Update set to ruynd automatically, or that you do frequent checks for updates.
The drastic solution, is of course to back up all data to a writable CD or two, then reformat and reinstall windows, update Windows to all new patches, add current antivirus, spyware and a firewall, and then make sure everything updates from then on.
* edit As Monkeybreath says, with 300+ viruses, its probably not worth faffing around. Bite the bullet, save data if you can (you do take regular backups dont you?), and start afresh with the drastic solution above so you dont get into this mess again.