Windows Vista is a complicated peice of Sh**. Issues that I have found with MIRC is that it moves the download draw to somewhere it shouldnt. It normally resides in c:/program files/mirc/download depending on the installation draw but with Microsoft creating many incompatibilities i will try to explain.
The reason you cannot see recieved files is that it is not there . YOu can get to your recieved files by going to c:\Users\'your user account'\AppData\Local\VirtualStore\Program Files\mIRC\download
Before doing this though you have to go to folder options and tell the computer to show hidden files and folders in the view options. Searching for the files will not help as they will not be displayed. This issues with MIRC reveals that alot of software will have certain issues with this new operating system. I hope this helps