Windows installer is infected. How do I get it running again?

I uninstalled my firewall, and something immediately attacked Norton antivirus, Spyware Doctor and Windows Installer, making them inoperable. It also hid my taskbar and start button. Most of my programs don't work and can not be engaged from Program Files. If I could get Windows Installer running again, I could download and run an uninfected antivirus. How can I do this, or how can I get rid of this virus or even identify it? I have Windows XP Pro.

Windows installer is infected. How do I get it running again?trojan

Sounds like spyware trojans. Try booting in Safe Mode by continually pressing F8 during the boot sequence. If you select Safe Mode with Networking you should be able to access the internet.

Free - For starters, download, update and run a full scan using SuperAntiSpyware (SAS) http://www.superantispyware.com/

SAS is an exceptionally easy to use application that will remove the majority of difficult Trojans and Worms, not just the easy ones. In addition it has the ability to remove Vundo and Smitfraud infections. SAS can lock your home page to prevent browser hi-jacking attempts.

If possible it is recommended that you load SAS in normal mode but run it in Safe Mode. Safe Mode can be started by continually pressing F8 during the boot sequence. If you are badly infected you might need to run SAS more than once to help clean your system. Afterwards if the only infections now being shown are in System Restore, once you are completely satisfied that you can boot normally, you can either delete the infected restore points manually, or temporarily turn off System Restore to purge the files.