To reply to threads in general, there is no restriction other than you must be a registered member (which you obviously are.)
However two forums here do have restrictions on who may reply or what a reply may contain. The Am I Infected? What Do I Do?
forum permits anyone to reply, however certain methods, tools, and advice are restricted to certain members or forbidden all together. Restrictions on the Am I Infected Forum can be found here: http://www.bleepingcomputer.com/forums/topic383782.html
The other forum with restrictions is the Malware Logs Analysis
forum. In this forum only members of the Malware Response Team are permitted to respond to topics (in addition to moderators and admins.)
These restrictions are not enforced by the forum software, but by staff. Posting a reply which runs afoul of the restrictions once will just earn you a note from a moderator not to do that again. Repeated infractions are dealt with in a progressively more harsh manner.
All other forums are wide open with the restriction that if someone is obviously infected with malware, you should point them to the Am I Infected or Malware Logs Analysis forum. Aside from that the rules are pretty standard: no spam, no flaming, no name calling, stay on topic, don't hijack someone else's thread, etc. The board rules
cover most of these.
If you are ever unsure whether you should reply to a particular topic, you are more than welcome to PM any of the forum staff
and ask; that's what we're here for!