I fired up the old XP Home system and watched System Explorer Performance window and 'MsMpEng.exe' used a bit of system resources to initialize but it has been quite ever since.
RAM usage is at 79% with IE8 with 5 tabs open.
Are you using the latest Windows Defender Antispyware: v1.57.85.0 available through its portal?
http://www.microsoft.com/security/portal I update it at least daily as it is updated at least as often as avast! definitions and sometimes more
Top Desktop Threats
1. Trojan:JS/Agent.FA
2. Worm:Win32/Conficker.B
3. TrojanDownloader:ASX/Wimad.BD
4. Virus:Win32/Sality.AM
5. TrojanDownloader:ASX/Wimad.AT
6. Virus:JS/Xilos
7. Worm:Win32/Roron.AA@mm
8. Virus:Win32/Mabezat.B
9. Worm:Win32/Hamweq!inf
10. Worm:Win32/Conficker.C