You can use ZoneAlarm to keep anyone from accessing your local hard drive from the LAN. Be sure to have it installed as a service so that it is operational even when you are not logged in.
Another way to prevent access would be to encrypt your data. You can use a product such as SafeHouse encryption to create a virtual disk that is encrypted at all times. The only time your data is in plaintext is when it is in memory or in the Windows Swap File. You can even put your Temp directories on the encrypted disk if you want to. Unless another user on the LAN has the SafeHouse software (not hard to get) and knows what encryption algorithm and encryption key you are using (should be hard to get or guess), they will not be able to access your data.
For more information on this topic, visit these other searchSecurity resources:
Best Web Links: Firewalls
Best Web Links: Encryption
This was first published in February 2002