I think i have a virus

I have windows xp- I also have microsoft security essentials and panda virus programs installed. The lat two weeks my computor has been getting slower and slower. Now it takes forever to open anything and the other day I had a thing pop up telling me I had all these trojan worms and all kinds of viruses on my computor and it was made to look like it was from Microsoft. The clicker was you had to purchase something to remove these. I fell for that one a year ago. I have tried to remove anything I thought may have been causing this. My computor will shut itself off after running so long even if you are in the middle of something. I have tried to run my scans but the computor will shut off after only 1/4 the way through scanning so I have no idea whether I have anything or not. I don't want to have to restore from beginning as I have a lot of pictures and music that I have already lost three times due to computor crashes. HELP!!!!!!!!!!!!!!!!

Please download Malwarebytes Anti-Malware from here.

Double Click mbam-setup.exe to install the application.

• Make sure a checkmark is placed next to Update Malwarebytes' Anti-Malware and Launch Malwarebytes Anti-Malware, then click Finish.
  
• If an update is found, it will download and install the latest version.
  
• Once the program has loaded, select "Perform Full Scan", then click Scan.
  
• The scan may take some time to finish,so please be patient.
  
• When the scan is complete, click OK, then Show Results to view the results.
  
• Make sure that everything is checked, and click Remove Selected.
  
• When disinfection is completed, a log will open in Notepad and you may be prompted to Restart. (See Extra Note)
  
• Please save the log to a location you will remember.
  
• The log is automatically saved by MBAM and can be viewed by clicking the Logs tab in MBAM.
  
• Copy and paste the entire report in your next reply.
  

Extra Note:

If MBAM encounters a file that is difficult to remove,you will be presented with 1 of 2 prompts,click OK to either and let MBAM proceed with the disinfection process. If asked to restart the computer, please do so immediately.