After every boot my computer shows found new hardware although nothing new is installed. But few days back I had updated the system. I don't know if that's the reason because there is few days gap between these events. I have vista 32bit, Intel core 2 Duo processor 2.10 Ghz.
Device manager shows an unknown device but when I click on reinstall driver windows cannot find anything.

Hello,
Are there any external devices connected to the computer?
If yes, disconnect the device and check if unknown device there or not.

no external device is connected

Where exactly do you see "unknown device"
Is it in device manager?

Also could you provide the make of your computer.

I use sony vaio . there's unknown device folder in device manager between network adapters and portable devices

Please provide the device ID for unknown device

Go to Device manager and right click on the unknown device and click properties-> go to details tab and please provide the Device ID.
In the dropdown menu, choose "Device Instance ID" to get the device ID.

Also,

Try to install Sony Corporation - Input - Sony Firmware Extension Parser Device from support website of sony and select the correct model number of the computer. http://esupport.sony.com/US/p/select-system.pl?