Windows Firewall is a good guess, so check that first. Did you port forward (in the router) previously before the fresh install? If so, it is possible the router is assigning your PC a new IP address and the forwarding isn't working anymore. If you weren't port forwarding, you should be, and it might have just been luck that it worked before.
uTorrent is what I use, and what most people use, but many of the newer versions are banned due to misreporting torrents, so if none of the router stuff is your problem and you just installed a fresh uTorrent, go back and find an older version that is approved by whatever sites you are using. I want to say I'm using something like 2.1.2 but that's just a guess since I'm at work. Point being though, they are well past version 3.0 now, so new isn't always better.
If none of that, you need to give us more details.