Can a hacker gain access to my computer and use it to access the internet through my ISP?

Short answer = yes.

How do I know if this is happening? My ISP received complaints about me regarding posts made from my ISP address on several message boards but I'm the only one who uses or has access to my home computer.

Wow, that's quite rare that the ISP get involved like that. Of course you should remove any unnecessary/unknown programs in Add/Remove Programs. Make sure you have an anti-virus program installed, up-to-date, and do a full scan of your hard drive. Do the same for spyware, Microsoft's free Anti-Spyware is very good. Make sure you have all the update Microsoft patches on your PC. (I'm assuming it's a Windows computer.)

This is an instance where it's hard to diagnose where the problem lies; I would only find the problem if I had access to the customer's PC. If you have a savvy techie friend who could troubleshoot the problem that would be great. In technical terms someone is using your computer as a proxy to go to websites - that way his IP is hidden and yours shows. If you remove the software that is allowing this then you are OK.

Thanks a bunch. I'm using spyware search and destroy right now to locate any unwanted software. And will be upgrading my firewall as well. My isp told me I could password protect my modem too. I'm hoping all these steps work.

Thanks a bunch. I'm using spyware search and destroy right now to locate any unwanted software. and will be upgrading my firewall as well. My isp told me I could password protect my modem too. I'm hoping all these steps work.
Do you have a wireless router? Then that is the most likely cause of the problem. I would remove the router at this point, connect your modem directly to the computer and read up on wireless security before rebuilding your wireless network.
Things to do:
- change admin password
- set encryption (WEP/WPA)
- use MAC filtering

How do people get this kind of access to my system and how can I prevent it in the future?

As Need says, if you have a wireless router, and you don't secure it properly, you are potentially letting anyone within wireless range use the internet through your account.

Your documentation with the wireless router (if you have one) (either manual or CD) will tell you how to setup wireless security.

How do people get this kind of access to my system and how can I prevent it in the future?

I hate to say it but 9/10 times, it's the users fault if the PC gets hacked. Some ways people could get access to your PC are:

By exploting bugs in software (e.g. MS Windows). Solution: Ensure you regularly use Windows Update to patch Windows.
From malware. This can be installed by visiting untrustworthy websites, viruses in emails. Solution: Install some good spyware protection, a virus scanner and a firewall.
Social engineering. You'll be surprised how many people will just not think and give their password out etc. Solution: NEVER tell anyone your password and try not to ever write it down.
Phishing. An email that appears valid but actually points you to some other, fake site. Enter your password on here etc. and it could cause you problems.


One note on this problem though. You said the forums were having trouble with somebody using your ISP address (I'm reading this as IP address). It is possible that it was somebody else, on a different computer with the same IP address you had used previously. ISP's assign IP addresses dynamically out of a pool of addresses. One week you could have a certain IP address, the next week someone else could have it. On a very busy forum, it's not unheard of to hit someone who's using an IP you previously used. For example, some times on Newgrounds if I go to vote on a submission, I will get an error telling me someone with my IP has already voted on this entry. I know it's not me or anyone using my PC.