What does the VARCHAR data look like? BIGINT does not allow decimals.
SQL's TIMESTAMP is really used to 'version stamp table rows'. Read up on this in the Books Online (BOL).
If your VARCHAR column is strictly integers, try this (change 'mycolumn' to whatever your column name is):
convert(bigint, mycolumn)
Refer to the BOL for more information on the various datatypes.
-SQLBill