jerryemerydelacruz, You will need to download and install the wireless drivers into the system as well as any other drivers for your computer. Below is a link to download drivers. There is also a link for driver install order. Drivers and Downloads-Vostro 1520 Drivers Help  For me to better assist in troubleshooting, please click my DELL-Jesse L and send me a Private Message with the Dell Service Tag number and your Dell Order number.  
I just did a clean install of win xp and now the wifi switch no longer activates wifi.  The ethernet connection, however, is working.  
Activate wifi on Vostro 1520