From the M$ link above:
. . .If the problem is still not resolved, reinstall Internet Explorer. If you are using the version of Internet Explorer that is included with your operating system, reinstall or repair your operating system. . . .

As for what caused the problem; my guess is one customer deleted some system files after an online virus scan reported the files uncleanable. - the topic above ended up only being part of the symptoms.

As for the other machine - not exactly sure. The customer "claims" it just started behaving that way. Virus scan came up clean, removed the usual spyware using Ad-aware and Spybot - nothing too much out of the norm. Still the problem persisted. However, they have a teenage son and Kazaa along with other P2P software was on the machine as well.