You're welcome 
1. Of those you may uninstall HijackThis thru add/remove programs and delete boot disk file on your desktop. I recommend keeping ATF Cleaner and running it occasionally to get rid of temporary items.
2. Depends on case. If trojan is backdoor type and steals personal details it's recommended to reformat.
3. Of free ones Antivir and Avast! are the best. I haven't used those you mentioned but Nod32 has done quite well in tests.
4. Of current versions I recommend Firefox. Good addons for it are Adblock Plus and NoScript.

1. Of those you may uninstall HijackThis thru add/remove programs and delete boot disk file on your desktop. I recommend keeping ATF Cleaner and running it occasionally to get rid of temporary items.
2. Depends on case. If trojan is backdoor type and steals personal details it's recommended to reformat.
3. Of free ones Antivir and Avast! are the best. I haven't used those you mentioned but Nod32 has done quite well in tests.
4. Of current versions I recommend Firefox. Good addons for it are Adblock Plus and NoScript.