Vista - Printer color profile onscreen display

I am trying to use Photoshop to display my images with the same colors as
they'll appear when I print them on my printer. I'm pretty new to color
management, and the first time I did it, I simply followed instructions and
everything worked great. I assigned my printer's ICC profile to the document
in Photoshop, and it looked onscreen almost exactly like it did when printed.

Then, a few weeks later, I tried again, and although I'm following the same
steps, the printout and the displayed images don't look the same any more.

I'm afraid I might have inadvertently changed some settings in Windows that
are preventing the monitor from properly displaying the color.

Does anyone know of any settings (services, startup programs, etc.) that
Windows might be using to apply color profiles to my display? Feel free to
share anything that you think might be remotely related.

I guess it's possible, although unlikely, since the computer is barely 2
months old, that I need to re-calibrate the monitor. In that case, does
anyone know any good (free or cheap preferred) software for calibrating
monitors?

Thanks!