“I read in a Help forum article that one can convert their disk for Windows XP to Windows XP SP3”
You do not need Rar to make a Windows XP Disk with SP3
A rar file is a compressed file format, which can contain one or more files.
The files are compressed and so take up less space than the originals, but can be uncompressed back to the original with no loss of data.
Because of their small size .rar files are often uses for storage or transmission of files
(Such as across the Internet)
RAR files are different to, but server a similar role to other compressed file formats such as Zip
How To: Slipstream your XP Disk With SP3