Open the IWAR directory on your hard drive. 
Inside there should be a folder named "PSG". 
Open it. Inside of the PSG folder there is 
a folder named "RESOURCE". Open this too. 
Inside of the "RESOURCE" folder there is a 
folder named "SCRIPTS". Inside of "SCRIPTS" 
there should be a folder named "PRECEDENCE". 
Inside of the "PRECEDENCE" folder there 
should be a file named "PREC" which you 
should open with a spreadsheet program, 
like Excel, and change all the numbers in 
the "PREC" column to 0, and save. Now you 
can access all the levels of IWAR from the 
beginning.