Problem with NAND gate in proteus
The 74S37 is a TTL logic IC. If you leave the inputs floating, then they will usually be seen as a logic 1, or the input will 'float' in the unknown region, where it could be a 1 or a 0. 

Proteus will use the red or blue logic indicators to show whether something is at a logic '1' (red) or logic '0' (blue). If you were to remove the multimeter, you will notice that the input logic indicators will go grey. This means they are undefined, although most TTL will tend to go high. 

For a NAND gate, use pull-up resistors on the inputs to tie them high, then you can drive the input low with your control signal. More on using resistors on logic gates can be found HERE. 

This is what I mean with proteus. You can see by leaving the input floating, they will go to an undefined state, which Proteus will mark as a grey logic indicator:





The voltmeter in Proteus has a modelled 10M(?) resistance which will allow the program to then register it as a logic 1, and the logic indicator will go red.