Michael mentions a grandmother as "Nana" in Goodbye, Toby. It can be reasonably assumed that he is referring to Barbara, as he also calls Barbara "Nana" in Season 5. Presuming this to be true, Barbara sends Michael a check every year on his birthday for fifty dollars. However, in recent years, she has been sending more than one check per year. Michael attributes this to her becoming more senile with age.
Michael went to her investment club to try and convince them to invest in The Michael Scott Paper Company. However, she declines, as she thinks the company will not succeed in the current economy. Although Michael mentions a grandmother in Season 4 that he believes is becoming senile, Barbara appears to be quite lucid in the episode in which she appears, which may indicate that she is not the same grandmother that Michael refers to in Season 4.