OpenOffice 2.0.1 vs MS Office

[scott at ehrlichtronics.com (Scott Ehrlich)]

I visited my in-laws to help them resolve a computer problem with their
Win XP Home machine, which also includes MS Office Small Busines Edition.

After fixing various things, I visited office.microsoft.com to try and
update Office.  Unfortunately, as the web site was trying to patch Office,
it responded with an error that it could not do the patching.

I figured Office would still work fine, and left for the night.

Much to my surprise, my mother-in-law emailed saying Excel no longer
worked.

I've tried various troubleshooting options with her via email - she is
computer-savvy.   Things are pointing to either an Office reinstall, or
possibly to OpenOffice.

If we/I went with OO, what compatibility issues should she expect to
encounter?   She also uses Quicken.   Don't know if that makes any
difference.

Thanks for any insight.

Scott