Your system itself has a certain amount of space reserved for system restore , in Vista it's 15% of your hard drive , can't remember off hand how much in XP but it's the same principle . Once this is full it overwrites the older ones .
I just Googled 'sychost' & apparently it's a virus . However there is also 'svchost' which is a vital part of your operating system . So are you sure you've spelt that right - not meaning to be patronising .
Where did you find 'sychost'? as the result of an anti-virus scan ?