Concatenate two columns based upon another column in SQL Server 2005

I have database as follows

A     B
1   ereee
2   ereee
2   sdfsd
3   nere
1   sdfsd

I want to have a data which will be as follows

A     B
1   ereee,sdfsd
2   ereee,sdfsd
3   nere

what is the query?