Password protect my Homenetwork

If XP is installed on NTFS-formatted partitions (the default, but you can still set up XP on FAT32 partitions, if one was so inclined), then you could adjust the NTFS Security permissions. Only allow your username access to the files themselves (or to the folder all files are in, if you keep them in the same folder.)
Then make sure you have the same user name AND password on both XP machines.
Always log out of both XP machines when you aren't using them.
Then if someone else logs into either one and tries to get to your files, the login name they are using is NOT yours, so they would not have any permissions on the files. Any access they would try would be denied.

humm ok, click start, navigate control panel, administrative tools, local security policy. Expand local policies, click on security options. Find Interactive logon Dont display last user name and password. enable that. Next is expand Account Policies, click on account lock out policy. Set that to 4 or 5 attempts before the account is locked out [more if you have a habit or forgetting]. The rest of the properties will automatically fill in e.g. lockout duration [default is half hour] Reset locked out after half hour [also default]. While your at it disable the quest account and rename your administrator account. And if your really paranoid, you'll rename the administrator account AND redo your user account to a power user account instead of the admin account. If anyone is trying to get in and uses your account, all they go is the power user account which minimizes the damage that can be done.

Now, next is click back and get to the control panel. Click on User Accounts, click on Change the way user log on and off. Uncheck the Use the Welcome Screen which will then use the "Classic" Log on which is a user name and password is required.