Here's a little single purpose program in AutoIt that will:
a) monitor a directory for .TIF or .TIFF files
b) call nconvert to output a PDF when it sees one
eg. test.tif -> test.pdf
Put it in a directory along with nconvert.exe, (available from here
Call it from the CLI, (you can close the CLI afterwards), like so: ncfet2p <dir>
The archive contains the executable, source, UDF and example.
Basically just a butchered version of the example script in the archive.
You can exit it using the context menu on the tray icon, (standard AutoIt icon).
NOTE: I tested it on XP using the x86 version of nconvert.exe but I don't see a reason why it shouldn't work on Vista/7 x86/x64.
Feel free to do what you like with it, change it to use IrfanView, (which I don't use), or something else but remember: If it formats your drive, I wasn't anywhere near it
Should mention, it will overwrite any existing PDF of the same name and it doesn't delete the original TIF.
UPDATE: Pops up a ToolTip for a few seconds when it start a conversion, (just to let you know it actually recognised a file).