My guess is that all of the MOSS accounts are not confirgured properly.

Your SQL database is full. This is the error  I always get when I run out of
space.
Check the Error log and it'll explain better.

SQL is not full..........................

MOSS worked for 1 month with no problems.

Problem is only with default.aspx page

Check the log files at C:\Program Files\Common Files\Microsoft Shared\web server
extensions\12\LOGS & Application event logs

Decrease logging severity level from Central Administration > Operations >
Diagnostic Logging to increase the number of events logged

I was able to fix the SharePoint sites  on the Prod SharePoint Server with
Microsoft. Apparently the default.aspx got corrupted and the only to way to fix
it was to make a copy of it, rename it and then change the Welcome start up page
for the site.

Is this happening for a site or a list? I had this problem with a list and
we had to recreate the list and dump the data from existing list. not a
agreeable solution but we used that as a work  around.

If it's happening for a site- try resetting the default.aspx using
sharepoint designer prior to that make a copy of the default.aspx because
when you reset the page , it will load the default settings.

For a site

resetting the default.aspx using
sharepoint designer HOW?

This is issues depanding upon your database full..

1, I think your installed for basic only. So basic installation it's
automatically invoke the Sql Server Express edition only. Default express
edition database size 4 GB. I think ur content database size is fulled. So u try
to create new content database.

The has to do with Symantec Protection for SharePoint 5.1 marking the
aspx files for the Welcome Page has having a virus when it doesn't