Wondering how to keep track of usage

I am getting ready to sign up with a company that only gives me 250GB per month. Is there a program that I can use on my desk top that will let me see how many GB I am using a month? Can not afford to go over the limit.

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft® Windows Vista™ Home Premium, Service Pack 2, 32 bit
Processor: Intel(R) Celeron(R) CPU 420 @ 1.60GHz, x64 Family 6 Model 22 Stepping 1
Processor Count: 1
RAM: 1014 Mb
Graphics Card: Intel(R) 82945G Express Chipset Family, 256 Mb
Hard Drives: C: Total - 142412 MB, Free - 64368 MB; D: Total - 10213 MB, Free - 2787 MB;
Motherboard: ELITEGROUP, 945GCT-M3
Antivirus: Norton Security with Backup, Updated and Enabled