[Technology columnist Walter S. Mossberg answers readers' questions]

Q: At work I am told to turn off my desktop PC at the end of the day. My wife's office tells her to leave her computer on overnight. My former employer said to turn off the monitor but leave the PC on. What's right?

A: Given the energy, environmental and economic crises, my view is that you should completely shut down both your computer and your monitor any time they will be unused for more than a few hours. And, of course, you should set them to go into sleep mode after they are idle for far less time than that. To be honest, I myself am not always faithful to that advice, but I am resolved to get better at it.

Of course, there are some nuances to the issue of overnight shutdowns. If you, or your company, routinely perform important operations overnight, like backups or security scans or system tune-ups, then you have to leave the computer on, but you should still turn off the monitor.

Q: How much memory do you recommend for a new computer running Vista Home Premium?

A: Two gigabytes minimum, and three gigabytes if your budget can handle it. The latter is roughly the maximum that popular consumer versions of Vista (called 32-bit versions) can use.

For smooth, quick operation in common tasks, it's been my experience that Vista demands more memory than either Windows XP or Apple's Leopard operating system. And the need for added memory rises if your PC uses 'shared' video memory, which means your graphics system lacks its own dedicated memory and relies on draining the main memory. This arrangement is common in less expensive computers.

Q: We are planning a trip to the British Isles this summer with a side trip to Brussels. We plan to bring our iPhone on the trip. What is the current situation with using European networks? Will my U.S. AT&T account just work or do I need to make pre-arrangements? I realize my iPhone won't be able to access the faster 3G networks over there, but can I use slower networks? Being limited to Wi-Fi would be a bummer.

A: The iPhone 3G can indeed handle 3G networks in Europe, though the original iPhone can handle only slower networks. Both models can use Wi-Fi there.

You will need to call AT&T before you leave, though, and 'provision,' or set up, your account, for international roaming. Note that such roaming is expensive, with very high rates for both voice use and data use. AT&T has some monthly add-on plans that cut these fees, but, if I were you, I would turn off data roaming until and unless I needed it; do most email and Web surfing via Wi-Fi; and consider downloading Truphone, a voice-calling program that uses Wi-Fi instead of cellphone networks.

专栏作家莫博士就有关电脑和其他电子设备的常见技术问题答读者问。

问：公司让我每天下班时关掉台式电脑。我妻子的公司叫她让电脑整夜开着。我以前的老板则要求关掉显示器但不关机。哪种做法才是对的？

答：从能源、环境和经济危机等方面的考虑，我的观点是在电脑超过几个小时不用的情况下，应该彻底关掉电脑和显示器。当然，在电脑闲置的时间远远少于几小时的情况下，你应该将它设成睡眠模式。说实话，我自己也并非总能坚持这个建议，但我决心在这方面有所改进。

当然，晚上要不要关机这个问题还有一些细节需要考虑。如果你或你的公司常常在晚上进行重要的操作，比如备份、安全扫描或系统升级，那你就得让电脑开着，但还是要关掉显示器。

问：对于运行Vista Home Premium版的新电脑，你推荐多大的内存？

答：最少2G，如果你预算能够承受的话可以达到3G。3G差不多是Vista通行的一些消费者版本（即所谓的32位版本）会用到的最大限度。

要流畅、迅速地运行常见的任务，我的经验是Vista比Windows XP或苹果(Apple)的Leopard操作系统需要更多的内存空间。而如果你的电脑使用的是集成显卡，也就是说你的显卡没有自己的专用内存，而是依靠主机内存，那就需要更多的内存空间。集成显卡在价格较低的电脑机型上很常见。

问：我们计划今年夏天去英国旅行，顺便去布鲁塞尔。我们打算带上iPhone。欧洲的手机网络现在是什么情况？我的美国电话电报公司(AT&T)帐户可以直接使用吗，还是得事先进行一些设置？我知道我的iPhone在欧洲可能不能使用速度较快的3G网络，但能用慢一些的网络吗？要是只能局限于使用无线Wi-Fi，那就太让人失望了。

答：iPhone 3G肯定能用于欧洲的3G网络，不过老版的iPone只能处理慢速网络。这两款机型都能使用欧洲的Wi-Fi网络。

不过你需要在出发前致电AT&T，并将你的帐户设成国际漫游。请注意这类漫游费用不菲，通话和数据流量收费都很高。AT&T有一些按月计费的附加套餐，会降低这些费用，不过，如果是我的话，除非真的需要用到，否则我会关掉数据漫游；大部分的电子邮件处理和上网都通过Wi-Fi网络进行；而且会考虑下载语音通话程序Truphone，使用Wi-Fi而不是手机网络。