Microsoft released "critical update" number KB835732 in April, 2004. This update causes a bug on some machines: the HOMEPATH environment variable is sometimes not set.
It's hard to know the exact conditions when this occurs, but it seems to be related to machines like laptops with "roaming profiles" that are sometimes disconnected from their networks. The symptoms are that programs that need to look at the HOMEPATH environment variable will fail.
The open source statistical package R (see www.r-project.org) versions up to 1.9.0 fail, because they detect the HOMEDRIVE environment variable and assume that (as documented) the HOMEPATH environment variable will also exist. The patched version of R (and releases from1.9.1 onwards) will no longer assume that this aspect of Microsoft documentation is correct.
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