Sometimes Sharepoint and Project are installed with a AD User instead of a system account and it happens that the password for the user expires after X days due to a AD Policy.
The best way to avoid this is to follow the following Technet article:
Plan for administrative and service accounts SharePoint Server
Change passwords used in administration accounts Project Server
However if you run into this issue this is where i've have refreshed the password in order to make the environment working again:
If you don't want to change the System account at the Farm level you could exclude the user from the password change and implement one at the Sharepoint level:http://technet.microsoft.com/en-us/library/ff724280.aspx