Have you got the latest drivers for your WLAN card, appropriate to your OS, installed?
If so then try downloading, installing and running a WiFi scanner program to see if there may be any interference causing you to lose signal.
Most WiFi routers default to Channels 1 or 6 so if there are WiFi routers nearby which are 'sharing' your channel and they have a strong enough signal it could be one reason. (By sharing I mean that they are all using the same channel for communications, not sharing your laptop or data. It's like having all the cars in one lane on a multi-lane road. It tends to slow down the traffic and can cause problems).
The scanner program will show you all the 'visible' networks in your area and the channel that they are using. Try altering the channel, in your router, that your network is using to a less crowded channel or even one that is not showing as being in use and see if it improves the stability of your connection.
Also have you got any electrical or electronic equipment close to the laptop which may cause electromagnetic interference when it starts up e.g. a fridge, a faulty CFL light, the ignitors on a gas stove etc.
The scanner program will also give you the signal level of your network and all the others as well. Remember that it is a negative value, so the higher the number the lower the signal level. You may find that moving your laptop location just a bit around may improve the signal level received and help to reduce the incidence of drop outs due to low signal.
Here is a link to just one WiFi scanner program. There are a lot more, just search for them online.