NOTE: This is assuming you're using WinXP.
1. Delete all your temp files. Start > Run > Type %temp% in the box & click OK. Delete all the files in the folder that opens (usually \Documents and Settings\your username\Local Settings\Temp or \Windows\TEMP).
2. Start > Run. Type msconfig in the box & click OK. In the System Configuration Utility (SCU) dialogue box that displays, select 'Selective Startup' & under it UNtick ALL the boxes except, 'Load System Services'
& 'Use Original BOOT.INI'. Click on Apply, OK. Reboot when prompted to do so.
3. When the bootup has completed, install N firewall the normal way.
4. On successful install (before you reboot), go back into the SCU (see above) & select 'Normal Startup - load all device drivers & services'. Click on Apply, OK. Reboot your PC.
If the above didn't work (or you're using an OS other than WinXP), please let us know with the necessary details. G