This can be a lot of things.
If you are using a router, then router has a firewall in it. Even if it is disabled you may not be able to host if you are using NAT due to partial implementation of NAT in most comsumer routers.
If you are behind a NAT router your ISP is using (very likely as most cable ISP use this scheme), they be having a problem with thier NAT setup.
Anti-virus programs, lack of free system memory can all contribute to this issue.