well, i need to come up with a way to supply power to my power inverter until my comptuer shuts down (30-60 seconds). is there a way I can build something for this, so when i turn my car off, a timer starts counting, and supplies power for a set time before it cuts power?

I would look into some sort of rechargeable UPS or maybe one of those jump start boxes that have an 12v outlet. I have a nice UPS at home for the occasional blackout. A common capacitor will help but discharges within about 10 seconds depending on the current draw. Why don't you just plan 30-60 seconds ahead and shut down.