From what you describe, your system integrity and security were almost certainly at least partially compromised. It sounds like there's at least some possibility that a malicious 3rd party is attempting to use your forum account. Unless you have a strong reason to believe that it's impossible that the 3rd party may have a chance at getting your password, you should fairly urgently change your password to something never previously used of reasonable complexity. For safety's sake, you should probably assume that all passwords previously used on the compromised system are at risk, including those which were only used before the malware incident (your system may have some passwords cached, so even those not used after the malware appeared are potentially at risk).
If you think there's a reasonable chance of identifying the malicious 3rd party, and feel motivated to do so, what you describe is probably at least a §1 offence under the
Computer Misuse Act 1990, possibly even §2 and §3 offences, carrying a prison term of up to 5 years on conviction. It's extraditable, but if the IP in question is outside the EU, US, and other UK-friendly countries, prosecution is probably a non-starter for anything other than a far more serious incident. The listed IP may also not be the true origin of the attack, as it's common for the more sophisticated attackers to proxy their attacks via previous compromised victim systems.
Very short version: Change your forum password to something completely new, and you should probably
change ALL passwords ASAP. You are probably a victim of a crime under UK law, but it may well not be worth the effort to try to pursue that if there's no other damage.
Sorry!
