Reformat issues

Hi guys.. I am using a dell optiplex 740... with windows 7 pro but i think i picked up a bug because i cant redownload dungeons and dragons online now... i have a windows 7 disk but it doesnt give me option to reformat... im thinking about using my old windows xp disk then reload windows 7 but i dont know what problems or challenges i wud run into.. any ideas?

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Professional, Service Pack 1, 64 bit
Processor: AMD Athlon(tm) 64 X2 Dual Core Processor 5000+, AMD64 Family 15 Model 75 Stepping 2
Processor Count: 2
RAM: 8126 Mb
Graphics Card: NVIDIA GeForce 6150 LE, 64 Mb
Hard Drives: C: Total - 152484 MB, Free - 96860 MB;
Motherboard: Dell Inc.,
Antivirus: avast! Antivirus, Updated and Enabled