query question

I am struggling on a query problem but it might be a piece of cake to you. Please help.

I have a table with three columns: Year | Month | Amount.

I would like my query to generate a result set with each row amount becomes accumulative which accumulate over years.

For example:

2005 | 11 | $400
2005 | 12 | $100
2006 | 1 | $200

The result set should be:

2005 | 11 | $400
2005 | 12 | $500
2006 | 1 | $700

Thanks alot