What model PC? What Operating System?  Required for ANY initial problem post.  It can make a difference.

In general, one thing that can cause this is malware/spyware.  HERE is a thread on that.

This can be a malware/spyware problem.  A "Rootkit" can cause this.  Run a complete malware/spyware scan and see if this finds anything.  If you don't have an up to date malware program, download and run the free Malwarebytes program.  You may have to run XP in Safe Mode to do the scan, depending on how bad it is. 

Malwarebytes

I had already checked for malware/spyware using malewarebytes but did it again in safe mode. Everything was OK. I decided to check services.msc again and decided to change the windows insaller from manual to automatic and that seems to have solved the problem. I hope permanently. Thanks for your help.