Accuracy of Regression Results?

I'm new to, really, all statistical models and have done some fair research specifically on regression models. I have come across some questions that I seem to have a hard time finding a clear answer to. When doing a regression model (I'm using Excels built in data analysis tool) I am curious how accurate it may be compared to a regression from R-Statistics. If not much is different or should be the same, where do I move going forward with the data received from the regression report? i.e. What should be done with variables that have low p-values? Thanks.
I haven't used the data analysis in excel, but you would look at residual plots to check for nonconstant variance, normally distributed error terms and other model adequacy checks.
Excel regression is accurate shows 5-6 digits which are correct.
Generally, there is no accuracy problem in regression's F statistic calculation, only matrix calculations and for p-value the F distribution in excel is accurate.