You will have two problems:
The best mic and best laptop in the world would still be unable to tell you how loud something is. It could do a spectrum analysis, but as for SPL, how would your laptop (or the software) know how sensitive your mic is to SPL? Think about the mic input on your laptop... you could set the gain anywhere and it wouldn't know.
In theory the software could let you calibrate the mic, but that would require you to have a special calibration noise generator.
The other problem you'll have is frequency... unless you have a mic specifically meant for calibration, your mic is likely not to have a flat response. So the SPL reading it would give for 1k, for example, wouldn't be the same as the reading it would give for pink noise, all other things being equal.
They make special spectrum analysis/SPL systems that consist of a calibrated mix and a card for your computer but their expensive and completely avoid your intention of using the equipment you already have.
That's my best answer!