Help! WindowsInstaller (5.0) Will Not Work

My windows installer (v.5.0) seems to be corrupted or something. I can't install or uninstall anything that uses .msi and i can't seem to find any way to fix this.

I want to uninstall OpenOffice, but get error that it can't find the .msi file. A search of my computer cannot find it either.

I tried to install VirtualBox (so I could run Win XP programs), but got Error: "The Windows Installer Service could not be accessed. this can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance", then "Sun Virtual Box Setup Wizard ended prematurely", then "Installation of MSI Failed. Error: 1603".

Tried to install Office (I believe the problem may have started with this). It seemed to install, but then I couldn't use it and couldn't uninstall it.

Other programs give me Windows Error 1719.
The Event Viewer log is full of Errors and Warnings (1,755 in less than that a lot?)
It seems that anything I try to install or uninstall that needs .msi will not work.
There is no download for Windows Installer 5.0 that I can find to repair or re-install. Have tried the Windows Installer Cleanup Utility.............doesn't work either.
Is there any way to fix this problem?

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