For Win95/98/ME:

Error: Can't Find System.ini 

Windows Won't Boot

Restart Computer. Press F8 Key repeatedly to get boot options Menu
Select Command Prompt Only

At Command Prompt type:
path=c:\windows\command
Hit Enter key then type:
scanreg /restore
Choose the most recent restore point when computer was working
Hit Enter key
Restart Computer to Boot Windows

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Start -> Run -> MsConfig -> System.ini

Select All in System.ini -> Apply -> Restart
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For Win XP:

Windows won't Boot 

Restart Computer. Press F8 Key repeatedly to get boot options Menu
Select safe mode

start -> run msconfig -> system restore

Choose the most recent restore point when computer was working:

Restart Computer to boot windows