Backup: It's crucial to perform a daily backup of your business data, and the backup logs must be reviewed to confirm that the backup is valid. I recommend at least two full weeks of backup to a media that can be stored off-site. It's important to use a tape drive cleaning cartridge regularly. Backups must be tested periodically by restoring data from backup.
Fault-Tolerance: RAID (a Redundant Array of Inexpensive Disks) is essential protection against server disk failure. This enables your server to continue running normally when a hard disk fails, so you can schedule a repair during non-business hours. For individual PCs, I recommend imaging the hard disk using Norton Ghost or Acronis True Image.
Power Protection: Battery backup, which enables computers to operate through power blackouts, is a necessity for servers, and will also improve the reliability of PCs. Surge protection is needed for every device and phone cable connected to your network. A surge entering through a monitor, printer, modem, scanner, hub, speakers, etc, can travel through your network cable and damage computers on your network.
Firewall: A hardware firewall, between the Internet and your network, is essential to keep hackers and viruses out of your computers. Some firewalls will filter junk mail or offensive content, or provide secure Virtual Private Network (VPN) connections to remote computers.
Anti-Virus: Because of the ubiquity of e-mail-borne viruses, anti-virus software is necessary on every Internet-connected computer. Virus definitions should be set to check daily for updates. All computers and servers should be scanned weekly for viruses. Most anti-virus vendors offer a business version with a network console to monitor the anti-virus status of all computers on the network.
Anti-Spyware: Adware and Spyware can slow PCs down to the point of uselessness, and may also steal passwords and confidential information. Most Anti-virus programs now include anti-spyware functionality; you don't need a separate program.
Anti-SPAM: Junk e-mail poses a serious threat to your business, potentially deceiving your employees into providing confidential information. A variety of solutions are available for your mail server, your Internet gateway, or individual PCs.
Error Logs: Windows event logs should be reviewed daily on servers, and weekly on individual PCs, to detect problems early.
Disk Maintenance: Temporary and deleted files should be purged regularly. Disks should be defragmented and checked for file system errors and available free space. Defragmentation can be automatically scheduled in Windows XP.
Software Updates: Critical Windows updates should be downloaded and applied automatically. The Windows Update website should be checked monthly for other applicable updates. Update Services can be installed on a server to update network computers more efficiently than having each individual PC download it's own updates.
I hope you find this information helpful. Please call if you'd like to discuss any of these issues in detail, or with questions or comments.