Well I was thinking if I have 4 GB of Ram on Win XP (x86) would the performance be better if I reduce or eliminate the amount of virtual memory (cause I think it uses a part of the HD for that).

If 4 GB of RAM is enough and knowing RAM is much faster than the HD, I guess you would get a performance increase by disabling or reducing the amount of virtual memory.

What do you think? Who could give me a good solution and explanation?

Disabling VM will not help. In fact it can prevent some programs from running. Most of the info put into VM is stuff windows is not using. If your playing a game, the game will not be in VM but other programs that don't need to be running can be put in VM freeing up more memory for your game. So in a way disabling VM makes your system use more physical memory then what it needs to.
Do a google search and your find many sites that have ran tests and have found no or very little benefit from disabling VM.

In task manager you can see what programs are using VM and how much. Goto view and select the columns you want.

You can't really see in the task manager how much is paged out. That virtual memory is not the same as the paging file.

VM will use the page file though for not-needed data. Thus if you disable the pagefile you force windows to keep in memory all the stuff that it doesn't need reducing the amount of memory available to other programs.

VM isn't the same as the page file, but many people do refer to them the same. Old habits die hard.