I searched on metalink, most of the time the problem was an incorrect ORA_NLS setting.

I also find a forum entry.
Here's what they say :

Subject: Failed to migrating a database from UNIX to NT

RDBMS Version: 8.0.5
Operating System and Version: Windows NT 4.0 / HP UX
Error Number (if applicable): IMP-00038
Product (i.e. SQL*Loader, Import, etc.): Import
Product Version: 8.0.5

Failed to migrating a database from UNIX to NT

I tried to import data from an exported dump which was produced by
one of our clients. The exported dump was a full database dump
from a HPUX box. Now, I want to load it into my NT machine and I received the error
IMP-00038: Could not convert to environment character set's handle.

The source server uses the default NLS_LANG and is an Oracle7.3.4
server. My Oracle uses the default NLS_LANG(AMERICA) and is an Oracle
8.0.5. The exported dump was saved to a CD and sent to us.

I read the Oracle Messages and codes manual and I could find any
explanation about the error IMP-00038.

Any idea?

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Subject: Re : Failed to migrating a database from UNIX to NT

There could be a couple of causes for this error.

Check your NT registry for the variables ORA_NLS33 and NLSRTL33 in Hkey_local_machine/software/oracle. These should be set to
%oracle_home\nlsrtl33\data and %oracle_home%\nlsrtl33 respectively.

My guess is that the UNIX database is using a 7-bit characterset and your NT database is 8-bit based on what would be the defaults. Set the registry variable NLS_LANG to the character set of the export dump
file. This is probably US7ASCII but you can check this by selecting from v$nls_parameters. This doesn't reflect the value of the NLS_LANG characterset but the characterset the database was created with.

Once you have imported the 7-bit characterset data, you should set your NLS_LANG back to the characterset of the database.

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I also looked at the registry off an NT server with ora 734.

key : HKEY_LOCAL_MACHINE\Software\ORACLE\ORA_NLS32
value : %ORACLE_HOME%\NLSRTL32\DATA

key : HKEY_LOCAL_MACHINE\Software\ORACLE\NLSRTL32
value : %ORACLE_HOME%\NLSRTL32

Hope this helps
Gert