mIRC does not limit your bandwidth (upload or download). Chances are that when you reinstalled Windows, you changed some setting (or the installation did) and that's the cause. Either that or the sender(s) have lowered their bandwidth or added additional slots or simply are sending faster to others such that you cannot get the full speed.
Keep in mind that DCC is far from the best download method. You're much better off using something that is meant for downloading, such as P2P programs or FTP or HTTP.
