Program Has Long Freeze While Writing Text File

I am making a program that loads, edits and saves 5000 rows x 100 columns of data. The data is stored in a datagridview. I use these commands to write the file:

dim z as integer
dim z1 as integer
fileopen(1,"c:drawermyfile.txt",openmode.output)
for z=0 to datagridview1.rows.count-1
[Code] .....

Every time I issue this command the whole program freezes until the computer is done writing the file. Is there some way I can get it to write this file without freezing up the program? Or, perhaps a better way to write this large data file?