There is also LogMeIn for Widows and Mac. You can install the LogMeIn Free client on the Windows/Mac machine. After that, add the same Windows/Mac machine to your free logmein.com account. Then, you can access the Windows machine remotely via any Web Browser. (Note: You need to have Flash plug-in installed on your Linux machine in order to access the remote machine).
The LogMeIn Free client has certain limitations as to what all you can do. These limitations can be overcome using the LogMeIn Pro client. Read: Paid.
There are many more LogMeIn products for a wide range of situations. You can check them out yourself: https://secure.logmein.com/products/default.aspx. There is also one product (LogMeIn Rescue) in which no software is required to be installed on the remote machine, but it is a paid version.