Sometimes generic Plug and Play monitor settings don't get the correct information through to the OS (especially if only a VGA connection is used) so as well as the latest Graphics drivers, you also need to install the latest monitor drivers from the monitor manufacturers website (more correctly termed a configuration file). If your monitors native resolution is 1600 x 900 then this should enable you to have that as your default.
I searched for your monitor but could only find a Samsung of that model - are you sure you have a Philips? The Samsung Monitor has a DVI connection which offers far superior performance to standard old D-Sub VGA so why the DVI-VGA converter? If it's because you don't have a DVI cable, then buy one - you won't regret it!
First of all sorry for the my fault. My monitor is SAMSUNG SyncMaster BX2031. But my monitor havent DVI port. I have only option to use DVI to VGA convertor.
The thing is ALL WORKED GOOD BEFORE I UPDATE THE BIOS. I have already installed the recent monitor driver form the samsung site. Please help me..
My monitor is SAMSUNG SyncMaster BX2031. But my monitor havent DVI port.
Then you actually have the BX2031N (the N at the end means without DVI Port). If you were sold this under the belief you were getting a BX2031 then get back to the seller and ask them to swap it for the correct model you ordered. If you had a DVI port equipped monitor you likely wouldn't be having this problem now and as already mentioned, their performance is superior in many ways.
If you are using the Magitune software, uninstall that using Control Panel. Have you simply tried powering off (fully - ie incl unplugging from mains) both the PC and monitor? Sometimes this can make a difference. You may also try resetting the monitor to it's default settings (check out its manual to find the correct button). I realise this problem has only arisen since you upgrade the BIOS but I very much doubt this is down to a problem with the BIOS update. After updating the BIOS did you go into the BIOS settings and load optimal defaults and save out of there? If not, try that too.