Ramoutar,
I've had a similar problem recently. Everything was running fine, but "suddenly" the Windows Installer messages showed up. It turned out to be caused by some trial software (Mindsoft's MindManager, in my case) I have installed and later removed using their uninstall script. It seems there was a bug in the installation/uninstall routine that overwrites/deletes some needed Windows software.
To resolve, I installed MindManager again, by which the missing routines became available again. I killed the home directory manually afterwards thus leaving some garbage in the Windows directory, but who cares?
Maybe something similar has happened on your machine as well. Hardest part will probably be to remember which software packages you installed/uninstalled and the go from there.
HTH, agrau