Microsoft recently released a security patch that makes it impossible to view CHM files that are stored on a network drive (as opposed to CHM Files stored on your own computer). The help file will appear, but instead of the topic text you will see this error message: "this page cannot be displayed". In some situations you might only see a blank white screen.
The security patch and its effects are fully described on Microsoft's web site at http://support.microsoft.com/kb/896358
If you trust all the computers on your LAN or intranet (and the people using them), you can lower the restrictions on the Local Intranet zone to allow CHM files to be displayed with the following steps:
1) Click Start, click Run, type regedit, and then click OK
2) Locate and then click on the following subkey:
Note: You will need to create this subkey if it does not exist.
3) Right click on the ItssRestrictions subkey, point to New and then click on the DWORD Value
4) Type MaxAllowedZone and then press enter
5) Right-click the MaxAllowedZone value and then click Modify
6) In the Value Data box, type 1, and then click OK
7) Quit Registry Editor.