Please do tha followings things...
1. What is the antivirus program installed on your computer?
2. Do you have a wired or wireless connection?
There is no method that you can let it reconnect automatically.
Here are few methods you can try to fix this issue.
Method 1:
Check if the issue occurs in ‘Safe Mode with Networking’. Here is an article that will give you information on ‘How to start computer in Safe Mode’:
http://windows.microsoft.com/en-US/windows7/Start-your-computer-in-safe-mode
If it works fine in ‘Safe Mode with Networking, there is a possibility that some of the third party service could be causing your network to drop. In most cases the Antivirus service.
Also set your computer to clean boot if it works fine in ‘Safe Mode’:
a. Click Start, type msconfig in the Start Search box, and then press ENTER.
b. On the General tab, click Selective Startup.
c. Under Selective Startup, click to clear the Load Startup Items check box.
d. Click the Services tab, click to select the Hide All Microsoft Services check box, and then click Disable All.
e. Click OK.
f. When you are prompted, click Restart.
g. After the computer starts, check if the issue is fixed.
If your issue is resolved after performing clean boot, then follow the steps mentioned in the following KB article to narrow down the exact source:
http://support.microsoft.com/kb/929135
Also, see the section on how return your computer to a Normal startup mode by following the steps under “Reset the computer to start as usual”, after performing the clean boot troubleshooting.
Note: This method would not be applicable if you have a wireless network. However, you can set the computer to ‘Clean boot’ and check it works fine.
Method 2:
Reset TCP/IP stack:
Here is an article that will help you with this:
http://support.microsoft.com/kb/299357
Note: This article applies for Windows Vista. However you may, Use a manual method to reset TCP/IP under the ‘Let me fixit myself’ section.
Thanks,