You can use free antivirus like Avira, Avg or Avast, they are as effective as full version antivirus if updated regularly and registered successfully.
You can download::
1. Avast Free
2. Avg Free
4. Avira Free
Don't install two antivirus at a time, if you are installing one of them or if there's any antivirus program installed on your computer, please uninstall and install the new one, otherwise you may have another problem with your problem.
First install Avast and run a full system scan, then check the scanned result, if you are sure that those trojans or virus etc..etc.. are removed, you are done, no need to uninstall and reinstall the other, or if you still doubt, you can try with the other two after the first one is uninstalled.
If you need further help, let me know.
Thanks for using FixYa.