Privilege Elevation Exception

Hi

I'm writing some code where the user should be system account.....like changing the user information list.

but this code has to run for each user ...for that I searched the internet and cud find below solution

{

{

{

//mycode

}}});

but his code throws me this exception

Request for the permission of type 'Microsoft.SharePoint.Security.SharePointPermission, Microsoft.SharePoint.Security, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c' failed.

Anyone has got some idea to handle this

My trust level is Medium

Thanks

Manu