Unable to install R package in Ubuntu 11.04

I'm new to Linux and R. I installed R 2.12 in Ubuntu 11.04. Today I tried to install a new package, so I ran the following command: install.packages('XML') But the installation failed and gave the following information: * installing *source* package ‘XML’ ... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking how to run the C preprocessor... gcc -E No ability to remove finalizers on externalptr objects in this verison of R checking for sed... /bin/sed checking for pkg-config... /usr/bin/pkg-config checking for xml2-config... no Cannot find xml2-config ERROR: configuration failed for package ‘XML’ * removing ‘/home/spirit/R/i686-pc-linux-gnu-library/2.12/XML’ The downloaded packages are in ‘/tmp/RtmpoZYxnv/downloaded_packages’ Warning message: In install.packages("XML") : installation of package 'XML' had non-zero exit status I also tried: sudo install.packages('XML') But it gives the same error information. Anyone can give me any advice?