After installing SP4 none of my ASP.NET pages developed using Framework 1.0 are showing the errors related to security

To resolve this issue, identify the user account that is used to run the program, and then assign the ‘Impersonate a client after authentication’ user right to that user account. To do this, follow these steps:

1. Click Start, point to Programs, point to Administrative Tools, and then click Local Security Policy.
2. Expand Local Policies, and then click User Rights Assignment.
3. In the right pane, double-click Impersonate a client after authentication.
4. In the Local Security Policy Setting dialog box, click Add.
5. In the Select Users or Group dialog box, click the user account that you want to add, click Add, and then click OK.
6. Click OK.

For more details refer Overview of the ‘Impersonate a Client After Authentication’….