This issue usually occurs on wireless network connections. I suggest that you turn off any security feature of wireless by following the steps below.
1. Click Start and access Control Panel.
2. Double-click on Network Connections
3. Right click on the wireless adapter and select Properties.
4. Choose Wireless Networks, click on your wireless network name and choose properties.
5. Under Network Authentication choose Open to turn off the wireless security.
6. Please try to connect.
If the issue persists, I suggest that you delete the connection, and recreate the wireless network connections. The following website may help you on creating the wireless connection.
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If the issue persists, I suggest that you reinstall the wireless adapter driver, and re-create the connection again.
Remove the adapter driver
1. Click Start. In the Start Search box type "devmgmt.msc" and press Enter.
2. Expand "Network Adapters", double-click on the entry of the wireless network device.
3. Click the tab "Driver", click "Uninstall".
4. Click Start. In the Start Search box type "appwiz.cpl" and press Enter.
5. Remove the entries related to the wireless network adapter.
Then reinstall the driver with your driver CD of from the manufacturer website. After installing, re-create the connection.
Also access the link and try to follow the steps to troubleshoot network related issues on Vista computer: