erm, start > run type services.msc press enter.
go through this list: http://www.blackviper.com/WinXP/servicecfg.htm
and make sure your services match....
if its just your start bar thats gone you can ctrl-alt-del and file>run and type explorer.exe
als re-run msconfig and put everything back to default, and go through the services to ensure the right ones are enabled.