Thankyou for your question.
While you can file with H&R block it's a good idea to ask before hand what this will cost and how experienced the particular person is who is going to help you.
(A friend recently used H&R block and while talking with the person taking care of them found that this person "wasn't quite through school yet" and my friend had of course thought that H&R used only seasoned tax consultants or at least retired accountants . )
A tax lawyer ,CPA or H&R block qualified specialist can help you best.
It's important to have "all your ducks in order" concerning your tax information before you go.
This can save you time and time is money at the tax lawyer .
All of your bank statements for that 6 yr period and receipts for expenses,interest paid,etc. are very important and can help simplify.
Be sure to talk money up front so you know what to expect as well as time element for your results.Get it in writing. This is something that you'll want to do whether it's a tax lawyer,accountant or H&R.
Also you may find you can do these taxes yourself and the IRS will let you know what penalty and taxes are after you've filed.
It's also important to understand that at this time of year it's really not the time to play catch up because the tax lawyers/CPA are really busy .
It is usually easier and cheaper for this to be done after April 15th.
Be sure to check out the link below so you are clear on the years you'll lose your refunds for by not filing at this time before making the final decision.
If you cannot do your taxes yourself now,at least file on this years taxes so you don't get a penalty /interest charge for the current year. Then get help as soon after "tax time" as you can.
Here's good information you may want to consider also.
http://taxes.about.com/od/taxtrouble/a/back_taxes.htm (this will help prepare you well)
Let me know please if I can be of further assistance? I'm here to help.