Have Antivirus Software Installed ( Norton, Trend Micro, Mcaffee)

Have Anti Spyware Installed ( Win Defender, Adaware)

Use Either A Hardware Or Software Firewall (routers usually have built in firewalls) Windows sp2 has a firewall built in.

Do All the Windows Updates

Make Sure Your Windows Is Genuine

Clean Out The Start Up

Run Hard Drive Clean Up And Tests Often

Run ScanDisk

Perform Backups Incase Of Major Malfunction