use strict;
use warnings;

use Inline C => DATA =>
           LIBS => '-luser32 -lkernel32 -lpsapi';

my $a = process_list();
map { print; print get_proc_name($_);print "\n"; } @{$a};


__END__
__C__

#include <windows.h>
#include <stdio.h>
#include <tchar.h>
#include <psapi.h>

char* get_proc_name( int processID )
{
    TCHAR szProcessName[MAX_PATH] = TEXT("<unknown>");


    HANDLE hProcess = OpenProcess( PROCESS_QUERY_INFORMATION |
                                   PROCESS_VM_READ,
                                   FALSE, processID );


    if (NULL != hProcess )
    {
        HMODULE hMod;
        DWORD cbNeeded;

        if ( EnumProcessModules( hProcess, &hMod, sizeof(hMod), 
             &cbNeeded) )
        {
            GetModuleBaseName( hProcess, hMod, szProcessName, 
                               sizeof(szProcessName)/sizeof(TCHAR) );
        }
    }

    CloseHandle( hProcess );
	return szProcessName;
}





SV* process_list(){

DWORD aProcesses[1024], cbNeeded, cProcesses;
    unsigned int i;
EnumProcesses( aProcesses, sizeof(aProcesses),  &cbNeeded );

cProcesses = cbNeeded / sizeof(DWORD);
AV* AV_procs = newAV();
    for ( i = 0; i < cProcesses; i++ )
    {
        if( aProcesses[i] != 0 )
        {
            av_push(AV_procs, newSViv( aProcesses[i] ) );
        }
    }
    return newRV_inc(AV_procs);
}



__OUTPUT__

C:\>perl test.pl
4P ?F >                       <==    strange line
292<unknown>
372<unknown>
432<unknown>

##</code><code>##


Haven't yet had a chance to take a thorough look at this.
I inserted the following near the beginning of your script:

use Inline C => Config =>
  BUILD_NOISY => 1;

That way you get to see any compiler warnings that are emitted during the C compilation, and with my cc-4.5.2 build of perl-5.14 I saw:

try_pl_f4a3.xs: In function 'get_proc_name':
try_pl_f4a3.xs:35:1: warning: function returns address of local variable
try_pl_f4a3.xs: In function 'process_list':
try_pl_f4a3.xs:57:5: warning: passing argument 2 of 'Perl_newRV' from incompatible pointer type
C:\MinGW\perl\lib\CORE/proto.h:2567:19: note: expected 'struct SV * const' but argument is of type 'struct AV *'

Whether there's anything relevant there, I don't know.

I get different output to you, of course. But in the middle of the output I also got one odd-looking line:

1348&#9508;aæ&#9787;&#8593;:ñ $   &#8593;:ñ  ~&#8593;&#9787;

When I switched to a gcc-3.4.5 build of perl-5.12, I found out what 1348 really was:
1348TomTomHOMERunner.exe

But then I got a strange line at the beginning of the output:

4ð¬&#9472;w4§Å &#8593;
whereas, with gcc-4.5.2, that had been:
4<unknown>