This 'misfeature' of SAV 10 made the roll out onto older systems particularly painful. Thankfully v10.1 does not do this by default but most copies of SAV out there are still v10.
Improving client restart performance
By default, the antivirus client software performs an Auto-Generated
QuickScan when the client computer restarts. QuickScan includes an
enhancement that scans for security risks such as adware and spyware.
This enhancement increases the time required to restart client computers,
and is not configurable from the Symantec System Center.
To improve client startup performance:
1. In the Symantec AntiVirus window, in the left pane, expand
Startup Scans, and then click Auto-Generated QuickScan.
2. In the right pane, click Edit.
3. In the Scan dialog box, on the Quick Scan tab, click Options.
4. In the Scan Options dialog box, under Scan Enhancements, uncheck
Scanning for traces of well-known viruses and Security Risks.
5. Click OK.
6. In the Scan dialog box, click OK.
Web connections will stall this way unless you are really running a web server sitting on 127.0.0.1:80 but it can help de-spyware-ize a box when needed. Just stick in %systemroot%/system32/drivers/etc.