Depends on what you mean by 'Idle".... if you have a lot of background apps running then the 25% certainly is in use and I wouldn't call it idle..... even though you may not be working on the rig.
The usage ought to be in the 2 to 4% range when literally idle.
"hazardous for my computer? "
Not exactly if it's doing some work.... but being busy all the time is certainly not a good sign and take into account the power consumption for doing nothing.... so it's basically time for you to check what all ought to be on in the programs and processes in your rig and make sure no useless bloatware or apps are running all the time....
On memory, Windows will allocate a fair amount of Memory for it's use when idle, as system requirements (ie as you open additional programs) it will release memory.
The only time you need to wory is under two conditions:
A) If your HDD show high usage when YOU are not writing/reading the HDD. This is because of low memory and windows ins using the HDD page file as ram. Only real effect is a decrease in performance. Can be solved by adding more ram.
B) you get an OUT OFF MEMORY error mesage. This is when you do NOT have enough physical RAM + Virual memory. Can Normally be solved by increasing the size of virual memory (HDD space allocated to the page file), and/or adding more physical memory.
Bottom line is Used vs free ram and it does not sound like you have a problem.
How much Ram (memory) do you have and which operating system are you using. Current recommend is 4 Gigs with 8 gigs the optimum. Ram is cheap these days, also more than 4 gigs of ram requires a 64 Bit operating system.