I have a big problem.. I need help!!!

I bought 11 month ago a Dell inspiron 1501 with Windows XP media center 2005 and it was getting slower, so I decided to buy a memory card of 1GB to increased performance and everything was good. However, after a while I noticed that my Dell was slow when starting the computer.. I mean to enter the system and I thought that was better to desinstall some programs that were installed but I did not use. Well turn out that for being so curios (and stupid) I made something wrong and the Help and support thing was not working and I thought was better to install the operating system again and everything went really bad, some programs were not working and some of them had to be installed again and then I thought the solution was on Dell support page and I downloaded by accident a BIOS thing and I installed it and now when I start my computer it goes a message that says that the system cannot recognize the battery and will not be charged by the and if I want to conti nue I have to press F2 twice and so on... I could "boot" (enter? sorry my english) to the system but I did not like to press all the time a key.. In short, dell sent me a new battery and now works but still is not the same computer anymore :( so my question is.. how do I put my Pc the was it was? Here the details of my system after installing the operating system CD that came with the notebook (sent by dell) Microsoft Windows XP Media Center Edition Version 2002 (In the Cd says it is media center version 2005 with updated rollup 2 no idea why is that) SP2 AMD Turion 64 Mobile Technology MK36 1.60GHz 1,37 GB of RAM (I should have more no?) Phiysical address extension. Please help Violeta