system restore points not being kept

my system is not keeping automatic or manual system restore points & i can't figure out why. i've checked the settings- it's on & i have it set to max space (are there other settings i should be aware about?).

it will keep the very last restore point (so like a few hours earlier in the day) but nothing before that. just today i created a manual restore point; & now after making a few changes here & there- my manual point is gone, although the last couple things i did are there as a restore point.

i don't know if it's related to a virus (everything looks clean), or maybe all my security or manintenance stuff is somehow erasing restore points, or a setting i'm missing.

if anyone else has this problem or can help- please!

win xp sp2
antivir antivirus
spywareguard
spywareblaster
sygate firewall
tuneup utilites 2007 (automatic tuneups)