During such a failure, can you still ping the server and what is the CPU usage,free system pages, available work items/server items, etc., etc. If you can't ping it than it's a network, nic, or cpu issue, relatively easy to fix. If it's not CPU then you'll have to look at the other items and see if/where it's running out of resources. I assume this is happening on more than one computer. Also when you ping it, ping it by name and by ip, are you using wins and/or dns, sometimes the name resolution server can be experiencing delays and it makes everything else delay. Just some thoughts...
virus issues but when u restart it will work…….
Note: you cant do this each an every time. keep a good purchased antivirus uptodate updates and enable autoscan & manual scan do it in-time