but i am facing the problem that whenever the card is locked camera boots only in preview mode. And in case i press function set + power key to startup my camera it hangs and shows clock for first few seconds then it hangs 
Why are you pressing set+power ? I don't personally have this camera, but I'm not aware of that being a shortcut in CHDK.
If I understood other posts correctly, you have to press shutter to get to record mode after booting CHDK. On some camera models, holding down the power button will take you directly to record.
On the SD870 (maybe on other Ixus models, too) you can do a "quick start" with this method:
- clock display time must be set to 0 in the Canon menu
- press & hold FUNC/SET, press POWER, wait until the clock display appear (~0.5s), then release FUNC/SET
The camera then starts directly in record mode...
@nouserid4meIf you want to use this method, do the following:
-> set "Clock Display" to "0s" (in the Canon menu)
-> hold FUNC/SET while you press POWER, release FUNC/SET directly after you see the clock screen - if you hold FUNC/SET too long, the camera crashes...
This "feature" is documented for a long time in the wikia:
CHDK for Dummies - Odd and Tricky cameras 
