ghetto way of checking if (outbound) tcp connections are blocked:
1. open up command prompt
2. type in "telnet bitseed.xf2.org 8333" (without quotes)
3. report what happens
alternate hostnames:
seed.bitcoin.sipa.be
dnsseed.bluematt.me
dnsseed.bitcoin.dashjr.org
All of them just idle at Connecting to (hostname)... then after 5 mins it comes up that it couldn't connect on port 8333: Connection failed. So is this my ISP or maybe router doing this?
Note: I've since completely reinstalled windows so as to remove any possible viruses.
You say you are using Microsoft Security essentials. There is only one sticky in technical support, and it is
No blocks downloaded - MS Security Essentials users please read.
I would go through any and all software firewall/virus scanners on the computer and disable them to isolate the problem. Likely something (installed on both computers) is not letting Bitcoin connections in or out. Alternately, if MSE isn't working, a virus may have disabled it and also broken network connectivity.
I've done everything they say to do in that thread, no change.