The latest version of SQL Server (2012) permits the following.

SELECT 
    RowID, 
    Col1,
    SUM(Col1) OVER(ORDER BY RowId ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS Col2
FROM tablehh
ORDER BY RowId

or

SELECT 
    GroupID, 
    RowID, 
    Col1,
    SUM(Col1) OVER(PARTITION BY GroupID ORDER BY RowId ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS Col2
FROM tablehh
ORDER BY RowId

This is even faster. Partitioned version completes in 34 seconds over 5 million rows for me.

Thanks to Peso, who commented on the SQL Team thread referred to in another answer.

Using a correlated query:

  SELECT t.id,
         t.count,
         (SELECT SUM(x.count)
            FROM TABLE x
           WHERE x.id <= t.id) AS cumulative_sum
    FROM TABLE t
ORDER BY t.id

Using MySQL variables:

  SELECT t.id,
         t.count,
         @running_total := @running_total + t.count AS cumulative_sum
    FROM TABLE t
    JOIN (SELECT @running_total := 0) r
ORDER BY t.id

Note:

• The JOIN (SELECT @running_total := 0) r is a cross join, and allows for variable declaration without requiring a separate SET command.
• The table alias, r, is required by MySQL for any subquery/derived table/inline view

Caveats:

• MySQL specific; not portable to other databases
• The ORDER BY is important; it ensures the order matches the OP and can have larger implications for more complicated variable usage (IE: psuedo ROW_NUMBER/RANK functionality, which MySQL lacks)