Something that you have running at startup is failing. You need to find out what it is.
First, always be sure the computer is virus/malware-free so you know you are working from a clean base.
http://www.elephantboycomputers.com/page2.html#Removing_Malware
If the machine is clean, manage your startup to find what program is failing.
Start Orb>Search box>type: msconfig and when it appears in the Results box above, right-click and choose "Run as Administrator".
If you are prompted for an administrator password or for a confirmation, type the password, or click Continue. Then see what is on the Startup tab. You don't need to restart immediately, but the next time you do you'll get a dialog saying you've used the Utility. Usually in Vista this will be blocked by Windows Defender and you'll need to allow it so you can then tick the box that says in effect, "don't bother me about this again".
Important - Do not use the System Configuration Utility to stop processes. Instead, use Services.
Start Orb>Search box>type: services and when it appears in the Results box above, right-click and choose "Run As Administrator". Confirm elevation as needed.
The free Autoruns program is very useful for managing your Startup - http://www.microsoft.com/technet/sysinternals/default.mspx
MS-MVP - Elephant Boy Computers - Don't Panic!