desktop microphone is not working properly

im facing typical problem with my desktop mic. my system -- pentium d, mb- intel 946 os xp professional sp ii and intex entry level sound card. when i use normal desktop mic with mic it does not response whereas when i connect larger mic through mixer it works fine.
some times when i speak through gmail or skype using normal mic initially i dont hear any sound from my mic but others can hear. typically after 2-3 min i too start getting feedback on my headphone. it continues till next 2-3 startups and after that initial situation appears. i hav opted for normal trouble shootings - didnt help.

i will be really greatful if somebody can solve this.

The driver for the cam nonexistent after SP1 of Vista, the driver for the microphone (Microsoft) does not start properly. For the driver part of the audio, unzip the latest driver for Windows XP with winrar. Manually update the driver by going to point in the unzipped files. For the micro part, the driver Usbaudio.sys Microsoft will not boot from SP1. Since, you are able to work for sometime, it is sure that something is wrong with the microphone or audio files. Still first try to uninstall and then reinstall the specific drivers and then try to check some another desktop microphone on you system for troubleshooting.