you havnt reset myboost back to its original value.
so it keeps subtracting from where it last stopped
you have commented out the line that stops myboost going below zero
MyBoost = MyBoost - 1 occurs after your if MyBoost <= 0 so even if you unrem the myboost = 0 it will still read -1
also
remove then from your if statement, then is only used for one line if statements.
place endif at the end of your if statement eg
if MyScore => 2000
set cursor 0,40
print "Press B For Boost"
set cursor 0,60
if Keystate(48)=1
MyBoost = MyBoost - 1
MySpeed = 20
print "Boost: ";(MyBoost/60)+1
if MyBoost <= 0
MyBoost = 0
MySpeed = 5
endif
endif
if keystate(48) = 0 then myboost = 17
endif