Not able to update

Hi,

I am new to ruby. OS: Windows XP Ruby Version --ruby 1.8.6 (2008-08-11 patchlevel 287) [i386-mswin32] When i run gem update --system after installing ruby

I am getting this error

C:\Documents and Settings\abhisheksreepal>gem update --system Updating RubyGems Updating rubygems-update ERROR: While executing gem ... (Gem::InstallError)     invalid gem format for C:/Ruby/lib/ruby/gems/1.8/cache/rubygems- update-1.3.5 .gem