Sharing Drive is excesive risk?

​ I have a home LAN. 4 PCs connected to WWW full time via firewalled cable modem. Each PC has XP SP2 and virus scan and its own firewall. ​

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​ I want to back up all 4 PCs to one external HD that is run off of the main PC.. I would use the XP provided Backup utility. ​

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​ To do this I need to set the drives in each PC to "Share". ​

​ Does this leave me more vulnerable to WWW mischief or not enough more to worry about. ​

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​ thanks! ​
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