Windows 2000 has an 'at' utility as well as a Scheduled Task feature. I'm not sure if the scheduled tasks can be as fine as every 5 seconds though. In my opinion, pinging every 5 seconds might be a bit excessive (and depending on your environment, might set off intrusion alarms). Perhaps sending a ping every couple of minutes might be a bit easier. Just a thought :-)
The Scheduled Task feature only allows a process to be run every one minute. I can't tell for sure but the at command looks to be the same. I will agree with the above comments that pinging every 5 seconds could degrade network performance if the server is critial enoungh to need to be checked so often.