Frankly, I'm not fond of IE7, though there are a lot people trying to crack the Windows Genuine Advantage to get a grasp of IE7. So, though I'm on Vista, which comes implicitly with IE7, I still opt for Mozilla Firefox.

Ya Firefox rocks! I simply can't live without some powerful and useful Firefox extensions, such as Web Developer, Firebug and SearchStatus.
The installation completed successfully for Firefox version 2.0.0.4.
I also managed to install a few must-have-but-also-wont-die-without-it extensions – Gestures, Google Toolbar, SearchStatus, Web Developer.
But, good hour doesn't persist. Right after the installation of the mentioned extensions, my Firefox was revenged by Windows. It hung aka not responding.
Then, of course I tried restarting Firefox. But, guess what I get?
The program must close to allow a previous installation attempt to complete. Please restart.
What the hell is this? As far as I'm concerned, I've already completed the required installation. What other previous installation attempt is going on?
No worry, we always have Dr. Google.
I pasted the string “The program must close to allow a previous installation attempt to complete. Please restart.” into Google search, and voila, I got the answer from MozillaZine Knowledge Base.
As explained in the article, there are a few possible factors causing anomaly in Firefox start-up. One of the known reasons, is titled Windows-specific problems. It has to do with Vista!
Luckily, the solution is simple. Simply browse to C:\Users\username\AppData\Local\VirtualStore\Program Files\Mozilla Firefox, and delete the file xpicleanup.dat will do.
Now, I'm with Firefox again.