I have readed every comment, some suggest do that, some said use this etc.
I have faced same kind of thing. I used a 89c2051 to switch On/Off a 35A relay, with 1A power supply. When ever relay is switch on mcu got hang. It is released by turning off the relay and reseting the mcu.
The main thing to remember here is mcu is the brain of your system. So if you keep it working and keep it off the load, every think will work fine, even in worst conditions.
I will suggest the following things.
1, Increase the pullup resistor to 4.7k or may be more (Like other have suggested).
2, used high base resistor connected with port. (may be Like other have suggested).
3, Use transistors with higher Beta value.
4, don't use 7805 or low drop. use following configuration.

D R Vcc |-------
Vcc -----|>|---//--o-----| 40
| |
----- |
C ----- |
| | MCU
| |
----- |
--- |
- |
|


Above is the circuit I have used. Value of R can be zero. D is diode like IN4001, while value of C was "4700 uf". Too keep it alive for worst conditions.