Itt would help other assist you if you would include your system setup.
O/S
ASP.Net Version
SQL version
IIS Version
Browser type and version
DNN version and whether it's local or remote install on a host
Computer info (type, memory, disk space etc)
Applications running (firewall, virus protection, etc)
It sounds more specific to your machine rather than DNNv4.0. VS2005, and SQL2005 default hardware configs require more resources, so depending on your setup, there could be some issues with loss of resources. So many variables, just saying my computer doesn't give folks anyplace to start, IMHO.
This is not just for your post, but many others are posting without providing any info, and it's almost impossible to zoom in on the real issue w/o some background...