Vista: Failure to create logon process

[Grimgoth]

I am having a new-ish problem with my vista home premium;
everything starts up hunky dory. However, after being left on for a while (read between 2-3 hours), things start slowing down, and some net access (via lan or wireless) starts not working. Another symptopm is that ctrl-alt-del fails to bring up the normal screen, but rather an error which is the failure to create logon process.

Any ideas on how to isolate the root of the problem?

 

[Grimgoth]

right, a further defining of the problem.
When Ctrl-alt-Del is pressed, the logon process fails to get or apply the security options.
que?