Acer 4520 Web camara enable

I am using ACER (Aspire 4520), with Windows Vista BASIC & then once i have format my laptop & install Vista Ultimate on it from that time my webcam is not appearing on desktop & I am enable to use web cam & another problem is when I have install yahoo massager that was installed on the time I start working all of the sudden it disappear plz provide me solution of this
with regard
pawan sharma

Please help me..! My acer 4520 webcam is doesn't work