Replace null with blank

UPDATE urTable
SET UrField = ''
WHERE urField IS NULL

update TableName
Set FieldName = ''
where FieldName = NULL

do it for all Comulmns

Is there a way to do all the columns in the table at the same time?

Sure,
UPDATE YourTableName
SET YourCol1 = '', YourCol2 = '', YourCol3 = '', ...
WHERE YourCol1 IsNull Or YourCol2 Is Null Or YourCol3 Is Null Or ...

warning: you can lose data with acperkins suggestion:  if YourCol1 has a non-null value but YourCol2 is null, the update statement will replace the value in YourCol1 with a blank (always run updates as a select first to see what it's going to do).  An alternative would be:

UPDATE YourTableName
SET YourCol1 = ISNULL(YourCol1,''), YourCol2 = ISNULL(YourCol2,''), YourCol3 = ISNULL(YourCol3,''), ...
WHERE YourCol1 IsNull Or YourCol2 Is Null Or YourCol3 Is Null Or ...

What you may be looking for is not allowing nulls:

-disallow nulls for each column in the design view of the table
-assign a default value of '' or 0 (all nulls will automatically be replaced)
-any new records will be free of nulls as well

this will ensure there are never nulls in your table and you won't ever have to deal with them!

MDD

Aneesh's solution is very good (I'd modify it to "SELECT DISTINCT COALESCE..."), coupled with the suggestions added by mdefehr (alter the table to add default values, et cetera).  I'd split points, 60/40.  This is just a comment.

Good point.

thanks guys